diff --git a/samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml b/samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml
index 769dbc0dace..0a5910f1aec 100644
--- a/samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml
+++ b/samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml
@@ -49,6 +49,11 @@
 
         <TextBlock Text="Validation Example"/>
         <CalendarDatePicker SelectedDate="{CompiledBinding ValidatedDateExample, Mode=TwoWay}"/>
+        
+        <TextBlock Text="Show WeekNumbers" />
+        <CalendarDatePicker IsWeekNumberVisible="True" 
+                            FirstDayOfWeek="Monday"
+                            WeekNumberRule="FirstFourDayWeek" />
       </StackPanel>
 
     </StackPanel>
diff --git a/samples/ControlCatalog/Pages/CalendarPage.xaml b/samples/ControlCatalog/Pages/CalendarPage.xaml
index 99727e91241..9da75587975 100644
--- a/samples/ControlCatalog/Pages/CalendarPage.xaml
+++ b/samples/ControlCatalog/Pages/CalendarPage.xaml
@@ -48,6 +48,12 @@
         <Calendar Name="BlackoutDatesCalendar"
                   SelectionMode="SingleDate" />
       </StackPanel>
+      <StackPanel>
+        <TextBlock Text="Weeknumbers"/>
+        <Calendar SelectionMode="SingleDate"
+                  IsWeekNumberVisible="True"
+                  WeekNumberHeader="CW" />
+      </StackPanel>
     </WrapPanel>
   </StackPanel>
 </ContentPage>
diff --git a/src/Avalonia.Controls/Calendar/Calendar.cs b/src/Avalonia.Controls/Calendar/Calendar.cs
index 7062a9af814..41b816cd573 100644
--- a/src/Avalonia.Controls/Calendar/Calendar.cs
+++ b/src/Avalonia.Controls/Calendar/Calendar.cs
@@ -1,4 +1,4 @@
-// (c) Copyright Microsoft Corporation.
+// (c) Copyright Microsoft Corporation.
 // This source is subject to the Microsoft Public License (Ms-PL).
 // Please see https://go.microsoft.com/fwlink/?LinkID=131993 for details.
 // All other rights reserved.
@@ -6,6 +6,7 @@
 using System;
 using System.Collections.ObjectModel;
 using System.Diagnostics;
+using System.Globalization;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
@@ -353,6 +354,57 @@ public IBrush? HeaderBackground
             set => SetValue(HeaderBackgroundProperty, value);
         }
 
+        /// <summary>
+        /// Defines the <see cref="IsWeekNumberVisible"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsWeekNumberVisibleProperty =
+            AvaloniaProperty.Register<Calendar, bool>(nameof(IsWeekNumberVisible));
+
+        /// <summary>
+        /// Gets or sets a value indicating whether week numbers are shown in the month view.
+        /// </summary>
+        public bool IsWeekNumberVisible
+        {
+            get => GetValue(IsWeekNumberVisibleProperty);
+            set => SetValue(IsWeekNumberVisibleProperty, value);
+        }
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberRule"/> property.
+        /// </summary>
+        public static readonly StyledProperty<CalendarWeekNumberRule> WeekNumberRuleProperty =
+            AvaloniaProperty.Register<Calendar, CalendarWeekNumberRule>(
+                nameof(WeekNumberRule),
+                defaultValue: (CalendarWeekNumberRule)DateTimeHelper.GetCurrentDateFormat().CalendarWeekRule);
+
+        /// <summary>
+        /// Gets or sets the rule used to determine the first week of the year for week number display.
+        /// The default is taken from the current culture. Use <see cref="CalendarWeekNumberRule.Iso"/>
+        /// for ISO 8601 week numbering.
+        /// </summary>
+        public CalendarWeekNumberRule WeekNumberRule
+        {
+            get => GetValue(WeekNumberRuleProperty);
+            set => SetValue(WeekNumberRuleProperty, value);
+        }
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberHeader"/> property.
+        /// </summary>
+        public static readonly StyledProperty<object?> WeekNumberHeaderProperty =
+            AvaloniaProperty.Register<Calendar, object?>(nameof(WeekNumberHeader), "#");
+
+        /// <summary>
+        /// Gets or sets the content displayed in the week-number column header cell.
+        /// Set this to a localized string such as <c>"CW"</c>, <c>"KW"</c>, or <c>"Wk"</c>
+        /// to give users context for the week-number column. Defaults to <c>"#"</c>.
+        /// </summary>
+        public object? WeekNumberHeader
+        {
+            get => GetValue(WeekNumberHeaderProperty);
+            set => SetValue(WeekNumberHeaderProperty, value);
+        }
+
         public static readonly StyledProperty<CalendarMode> DisplayModeProperty =
             AvaloniaProperty.Register<Calendar, CalendarMode>(
                 nameof(DisplayMode),
@@ -2209,6 +2261,9 @@ static Calendar()
             DisplayDateProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateChanged(e));
             DisplayDateStartProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateStartChanged(e));
             DisplayDateEndProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateEndChanged(e));
+            IsWeekNumberVisibleProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
+            WeekNumberRuleProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
+            WeekNumberHeaderProperty.Changed.AddClassHandler<Calendar>((x, _) => x.UpdateMonths());
             KeyDownEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyDown(e));
             KeyUpEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyUp(e));
         }
diff --git a/src/Avalonia.Controls/Calendar/CalendarItem.cs b/src/Avalonia.Controls/Calendar/CalendarItem.cs
index e33298023f0..09cfac405bd 100644
--- a/src/Avalonia.Controls/Calendar/CalendarItem.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarItem.cs
@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Primitives
     [TemplatePart(PART_ElementNextButton,     typeof(Button))]
     [TemplatePart(PART_ElementPreviousButton, typeof(Button))]
     [TemplatePart(PART_ElementYearView,       typeof(Grid))]
-    [PseudoClasses(":calendardisabled")]
+    [PseudoClasses(":calendardisabled", ":hasweeknumbers")]
     public sealed class CalendarItem : TemplatedControl
     {
         /// <summary>
@@ -49,6 +49,9 @@ public sealed class CalendarItem : TemplatedControl
 
         private readonly System.Globalization.Calendar _calendar = new GregorianCalendar();
 
+        private CalendarWeekNumberLabel[] _weekNumberLabels = Array.Empty<CalendarWeekNumberLabel>();
+        private CalendarWeekNumberLabel? _weekNumberHeaderLabel;
+
         internal Calendar? Owner { get; set; }
         internal CalendarDayButton? CurrentButton { get; set; }
 
@@ -168,16 +171,18 @@ private void PopulateGrids()
         {
             if (MonthView != null)
             {
-                var childCount = Calendar.RowsPerMonth + Calendar.RowsPerMonth * Calendar.ColumnsPerMonth;
+                // +7 for the week-number column (1 header + 6 data cells)
+                var childCount = Calendar.RowsPerMonth + Calendar.RowsPerMonth * Calendar.ColumnsPerMonth + Calendar.RowsPerMonth;
                 using var children = new PooledList<Control>(childCount);
 
+                // Day-of-week title cells — day columns are at 1-7 (shifted right by 1)
                 for (int i = 0; i < Calendar.ColumnsPerMonth; i++)
                 {
                     if (DayTitleTemplate?.Build() is Control cell)
                     {
                         cell.DataContext = string.Empty;
                         cell.SetValue(Grid.RowProperty, 0);
-                        cell.SetValue(Grid.ColumnProperty, i);
+                        cell.SetValue(Grid.ColumnProperty, i + 1);
                         children.Add(cell);
                     }
                 }
@@ -198,7 +203,7 @@ private void PopulateGrids()
                             cell.Owner = Owner;
                         }
                         cell.SetValue(Grid.RowProperty, i);
-                        cell.SetValue(Grid.ColumnProperty, j);
+                        cell.SetValue(Grid.ColumnProperty, j + 1);
                         cell.CalendarDayButtonMouseDown += cellMouseLeftButtonDown;
                         cell.CalendarDayButtonMouseUp += cellMouseLeftButtonUp;
                         cell.PointerEntered += cellMouseEntered;
@@ -206,7 +211,26 @@ private void PopulateGrids()
                         children.Add(cell);
                     }
                 }
-                
+
+                // Week-number cells — added after existing cells to preserve child indices 0–48.
+                // Header placeholder (row 0, col 0)
+                _weekNumberHeaderLabel = new CalendarWeekNumberLabel();
+                _weekNumberHeaderLabel.IsHeader = true;
+                _weekNumberHeaderLabel.SetValue(Grid.RowProperty, 0);
+                _weekNumberHeaderLabel.SetValue(Grid.ColumnProperty, 0);
+                children.Add(_weekNumberHeaderLabel);
+
+                // Data labels (rows 1–6, col 0)
+                _weekNumberLabels = new CalendarWeekNumberLabel[Calendar.RowsPerMonth - 1];
+                for (int i = 1; i < Calendar.RowsPerMonth; i++)
+                {
+                    var label = new CalendarWeekNumberLabel();
+                    label.SetValue(Grid.RowProperty, i);
+                    label.SetValue(Grid.ColumnProperty, 0);
+                    _weekNumberLabels[i - 1] = label;
+                    children.Add(label);
+                }
+
                 MonthView.Children.AddRange(children);
             }
 
@@ -254,6 +278,10 @@ protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
             NextButton = e.NameScope.Find<Button>(PART_ElementNextButton);
             MonthView = e.NameScope.Find<Grid>(PART_ElementMonthView);
             YearView = e.NameScope.Find<Grid>(PART_ElementYearView);
+
+            // Prepend a column for week numbers; day columns follow in positions 1–7.
+            // Done here so PopulateGrids() can place cells at the correct indices.
+            MonthView?.ColumnDefinitions.Insert(0, new ColumnDefinition(GridLength.Auto));
             
             if (Owner != null)
             {
@@ -373,6 +401,7 @@ internal void UpdateMonthMode()
             {
                 SetDayTitles();
                 SetCalendarDayButtons(_currentMonth);
+                UpdateWeekNumberLabels(_currentMonth);
             }
         }
         private void SetMonthModeHeaderButton()
@@ -589,6 +618,36 @@ private void SetCalendarDayButtons(DateTime firstDayOfMonth)
             }
         }
 
+        private void UpdateWeekNumberLabels(DateTime firstDayOfMonth)
+        {
+            if (_weekNumberLabels.Length == 0)
+                return;
+
+            int lastMonthToDisplay = PreviousMonthDays(firstDayOfMonth);
+            DateTime firstDateDisplayed = DateTimeHelper.CompareYearMonth(firstDayOfMonth, DateTime.MinValue) > 0
+                ? _calendar.AddDays(firstDayOfMonth, -lastMonthToDisplay)
+                : firstDayOfMonth;
+
+            bool show = Owner?.IsWeekNumberVisible ?? false;
+            var rule = Owner?.WeekNumberRule ?? (CalendarWeekNumberRule)DateTimeHelper.GetCurrentDateFormat().CalendarWeekRule;
+            var firstDayOfWeek = Owner?.FirstDayOfWeek ?? DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek;
+
+            PseudoClasses.Set(":hasweeknumbers", show);
+
+            for (int i = 0; i < _weekNumberLabels.Length; i++)
+            {
+                DateTime firstDayOfRow = _calendar.AddDays(firstDateDisplayed, i * NumberOfDaysPerWeek);
+                _weekNumberLabels[i].Content = DateTimeHelper.GetWeekOfYear(firstDayOfRow, rule, firstDayOfWeek).ToString();
+                _weekNumberLabels[i].IsVisible = show;
+            }
+
+            if (_weekNumberHeaderLabel != null)
+            {
+                _weekNumberHeaderLabel.Content = Owner?.WeekNumberHeader;
+                _weekNumberHeaderLabel.IsVisible = show;
+            }
+        }
+
         internal void UpdateYearMode()
         {
             if (Owner != null)
diff --git a/src/Avalonia.Controls/Calendar/CalendarWeekNumberLabel.cs b/src/Avalonia.Controls/Calendar/CalendarWeekNumberLabel.cs
new file mode 100644
index 00000000000..c1b6fc27d4b
--- /dev/null
+++ b/src/Avalonia.Controls/Calendar/CalendarWeekNumberLabel.cs
@@ -0,0 +1,27 @@
+namespace Avalonia.Controls.Primitives
+{
+    /// <summary>
+    /// Displays a week number in the month view of a <see cref="Calendar"/>.
+    /// Apply styles targeting <see cref="CalendarWeekNumberLabel"/> to customise
+    /// the appearance — for example <c>FontWeight="Bold"</c>.
+    /// Use the <c>:header</c> pseudo-class to target the column header cell (row 0).
+    /// </summary>
+    public sealed class CalendarWeekNumberLabel : ContentControl
+    {
+        /// <summary>
+        /// Gets or sets a value indicating whether this label is the column header cell
+        /// (placed in row 0 of the month grid, above the week-number data cells).
+        /// Themes can target this with the <c>:header</c> pseudo-class.
+        /// </summary>
+        public bool IsHeader
+        {
+            get => field;
+            internal set
+            {
+                if (field == value) return;
+                field = value;
+                PseudoClasses.Set(":header", value);
+            }
+        }
+    }
+}
diff --git a/src/Avalonia.Controls/Calendar/CalendarWeekNumberRule.cs b/src/Avalonia.Controls/Calendar/CalendarWeekNumberRule.cs
new file mode 100644
index 00000000000..11646be9a95
--- /dev/null
+++ b/src/Avalonia.Controls/Calendar/CalendarWeekNumberRule.cs
@@ -0,0 +1,37 @@
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Defines the rule that determines the first week of the calendar year for week-number display.
+    /// </summary>
+    public enum CalendarWeekNumberRule
+    {
+        /// <summary>
+        /// The first week of the year starts on the first day of the year and ends before the
+        /// following designated first day of the week. Equivalent to
+        /// <see cref="System.Globalization.CalendarWeekRule.FirstDay"/>.
+        /// </summary>
+        FirstDay = 0,
+
+        /// <summary>
+        /// The first week of the year begins on the first occurrence of the designated first day
+        /// of the week on or after the first day of the year. Equivalent to
+        /// <see cref="System.Globalization.CalendarWeekRule.FirstFullWeek"/>.
+        /// </summary>
+        FirstFullWeek = 1,
+
+        /// <summary>
+        /// The first week of the year is the first week with four or more days before the
+        /// designated first day of the week. Equivalent to
+        /// <see cref="System.Globalization.CalendarWeekRule.FirstFourDayWeek"/>.
+        /// </summary>
+        FirstFourDayWeek = 2,
+
+        /// <summary>
+        /// Uses ISO 8601 week numbering: the first week of the year must contain at least four days,
+        /// and Monday is treated as the first day of the week, regardless of
+        /// <see cref="Calendar.FirstDayOfWeek"/>. This is the most common rule in European locales
+        /// and professional calendar applications.
+        /// </summary>
+        Iso = 3,
+    }
+}
diff --git a/src/Avalonia.Controls/Calendar/DateTimeHelper.cs b/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
index 570f05cfe8d..af9ee79a22d 100644
--- a/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
+++ b/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
@@ -131,5 +131,27 @@ public static string ToYearString(DateTime date)
             var format = GetCurrentDateFormat();
             return date.Year.ToString(format);
         }
+
+        /// <summary>
+        /// Gets the week-of-year number for the specified date.
+        /// </summary>
+        /// <param name="date">The date.</param>
+        /// <param name="rule">The rule that defines what constitutes the first week of the year.</param>
+        /// <param name="firstDayOfWeek">The first day of the week. Ignored when <paramref name="rule"/> is <see cref="CalendarWeekNumberRule.Iso"/>.</param>
+        /// <returns>The week number that <paramref name="date"/> falls in.</returns>
+        public static int GetWeekOfYear(DateTime date, CalendarWeekNumberRule rule, DayOfWeek firstDayOfWeek)
+        {
+            var mappedRule = (CalendarWeekRule)(int)rule;
+
+            // Iso is an explicit shorthand for ISO 8601 (FirstFourDayWeek + Monday).
+            // Also catches FirstFourDayWeek + Monday explicitly, because .NET's
+            // Calendar.GetWeekOfYear incorrectly returns week 53 for late-December dates
+            // that ISO 8601 assigns to week 1 of the next year (e.g. 2018-12-31).
+            if (rule == CalendarWeekNumberRule.Iso ||
+                (mappedRule == CalendarWeekRule.FirstFourDayWeek && firstDayOfWeek == DayOfWeek.Monday))
+                return ISOWeek.GetWeekOfYear(date);
+
+            return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, mappedRule, firstDayOfWeek);
+        }
     }
 }
diff --git a/src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.Properties.cs b/src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.Properties.cs
index 864ba86ef08..0ef6a73a2ff 100644
--- a/src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.Properties.cs
+++ b/src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.Properties.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Globalization;
 using Avalonia.Controls.Primitives;
 using Avalonia.Data;
 using Avalonia.Layout;
@@ -140,6 +141,24 @@ public partial class CalendarDatePicker
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             ContentControl.VerticalContentAlignmentProperty.AddOwner<CalendarDatePicker>();
 
+        /// <summary>
+        /// Defines the <see cref="IsWeekNumberVisible"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsWeekNumberVisibleProperty =
+            Calendar.IsWeekNumberVisibleProperty.AddOwner<CalendarDatePicker>();
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberRule"/> property.
+        /// </summary>
+        public static readonly StyledProperty<CalendarWeekNumberRule> WeekNumberRuleProperty =
+            Calendar.WeekNumberRuleProperty.AddOwner<CalendarDatePicker>();
+
+        /// <summary>
+        /// Defines the <see cref="WeekNumberHeader"/> property.
+        /// </summary>
+        public static readonly StyledProperty<object?> WeekNumberHeaderProperty =
+            Calendar.WeekNumberHeaderProperty.AddOwner<CalendarDatePicker>();
+
         /// <summary>
         /// Gets a collection of dates that are marked as not selectable.
         /// </summary>
@@ -358,5 +377,26 @@ public VerticalAlignment VerticalContentAlignment
             get => GetValue(VerticalContentAlignmentProperty);
             set => SetValue(VerticalContentAlignmentProperty, value);
         }
+        
+        /// <inheritdoc cref="Calendar.IsWeekNumberVisible"/>
+        public bool IsWeekNumberVisible
+        {
+            get => GetValue(IsWeekNumberVisibleProperty);
+            set => SetValue(IsWeekNumberVisibleProperty, value);
+        }
+
+        /// <inheritdoc cref="Calendar.WeekNumberRule"/>
+        public CalendarWeekNumberRule WeekNumberRule
+        {
+            get => GetValue(WeekNumberRuleProperty);
+            set => SetValue(WeekNumberRuleProperty, value);
+        }
+
+        /// <inheritdoc cref="Calendar.WeekNumberHeader"/>
+        public object? WeekNumberHeader
+        {
+            get => GetValue(WeekNumberHeaderProperty);
+            set => SetValue(WeekNumberHeaderProperty, value);
+        }
     }
 }
diff --git a/src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml b/src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml
index 2613a0a1334..03fcc462dac 100644
--- a/src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml
+++ b/src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml
@@ -402,6 +402,8 @@
       <StaticResource x:Key="CalendarViewBorderBrush" ResourceKey="SystemControlForegroundChromeMediumBrush" />
       <StaticResource x:Key="CalendarViewWeekDayForegroundDisabled"
                       ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
+      <StaticResource x:Key="CalendarViewWeekNumberSeparatorForeground" 
+                      ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonBackground" ResourceKey="SystemControlTransparentBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonForegroundPointerOver"
                       ResourceKey="SystemControlForegroundBaseHighBrush" />
@@ -1228,6 +1230,8 @@
       <StaticResource x:Key="CalendarViewBorderBrush" ResourceKey="SystemControlForegroundChromeMediumBrush" />
       <StaticResource x:Key="CalendarViewWeekDayForegroundDisabled"
                       ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
+      <StaticResource x:Key="CalendarViewWeekNumberSeparatorForeground" 
+                      ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonBackground" ResourceKey="SystemControlTransparentBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonForegroundPointerOver"
                       ResourceKey="SystemControlForegroundBaseHighBrush" />
diff --git a/src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml b/src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml
index 395e2d514f6..7e44c412a67 100644
--- a/src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml
+++ b/src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml
@@ -133,7 +133,10 @@
                           SelectedDate="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedDate, Mode=TwoWay}"
                           DisplayDate="{TemplateBinding DisplayDate}"
                           DisplayDateStart="{TemplateBinding DisplayDateStart}"
-                          DisplayDateEnd="{TemplateBinding DisplayDateEnd}" />
+                          DisplayDateEnd="{TemplateBinding DisplayDateEnd}"
+                          IsWeekNumberVisible="{TemplateBinding IsWeekNumberVisible}"
+                          WeekNumberRule="{TemplateBinding WeekNumberRule}"
+                          WeekNumberHeader="{TemplateBinding WeekNumberHeader}" />
               </Popup>
             </Grid>
           </Panel>
diff --git a/src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml b/src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml
index ac3ebadbb80..4b0fb20195a 100644
--- a/src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml
+++ b/src/Avalonia.Themes.Fluent/Controls/CalendarItem.xaml
@@ -73,9 +73,10 @@
           <!-- To keep calendar from resizing when switching DisplayMode
                  In WinUI Min-Width from TemplateSettings
                  basically...MinWidth of DayItem = 40, 40 * 7 = 280 + margins/padding = ~294
+                 With week numbers: 40 * 8 = 320 + margins/padding = ~334 (see :hasweeknumbers style below)
                  Viewport height is set from # of rows displayed (2-8) in Month mode, = ~290 for 6 weeks (+ day names)
                  -->
-          <Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" RowDefinitions="40,*" MinWidth="294">
+          <Grid x:Name="LayoutRoot" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" RowDefinitions="40,*" MinWidth="294">
             <Grid ColumnDefinitions="5*,*,*">
               <Button Name="PART_HeaderButton"
                       Theme="{StaticResource FluentCalendarButton}"
@@ -159,5 +160,12 @@
     <Style Selector="^ /template/ Button:pressed > Path">
       <Setter Property="Stroke" Value="{DynamicResource CalendarViewNavigationButtonForegroundPressed}" />
     </Style>
+    <!-- When week numbers are visible, the MonthView gains an extra 40px column.
+         Increase the LayoutRoot MinWidth to prevent the calendar from shrinking
+         when switching to year/decade view (which does not have that extra column).
+         334 = 40 (week-number col) + 294 (7 day-cols + margins/padding) -->
+    <Style Selector="^:hasweeknumbers /template/ Grid#LayoutRoot">
+      <Setter Property="MinWidth" Value="334" />
+    </Style>
   </ControlTheme>
 </ResourceDictionary>
diff --git a/src/Avalonia.Themes.Fluent/Controls/CalendarWeekNumberLabel.xaml b/src/Avalonia.Themes.Fluent/Controls/CalendarWeekNumberLabel.xaml
new file mode 100644
index 00000000000..c458f355ca6
--- /dev/null
+++ b/src/Avalonia.Themes.Fluent/Controls/CalendarWeekNumberLabel.xaml
@@ -0,0 +1,39 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+  <ControlTheme x:Key="{x:Type CalendarWeekNumberLabel}" TargetType="CalendarWeekNumberLabel">
+    <Setter Property="MinWidth" Value="40" />
+    <Setter Property="MinHeight" Value="40" />
+    <Setter Property="Padding" Value="0" />
+    <Setter Property="Foreground" Value="{DynamicResource CalendarViewOutOfScopeForeground}" />
+    <Setter Property="HorizontalContentAlignment" Value="Center" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
+    <Setter Property="FontSize" Value="{DynamicResource CalendarDayButtonFontSize}"></Setter>
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Border BorderBrush="{DynamicResource CalendarViewWeekNumberSeparatorForeground}"
+                BorderThickness="0,0,1,0">
+          <ContentPresenter Name="PART_ContentPresenter"
+                            ContentTemplate="{TemplateBinding ContentTemplate}"
+                            Content="{TemplateBinding Content}"
+                            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                            Margin="{TemplateBinding Padding}" />
+        </Border>
+      </ControlTemplate>
+    </Setter>
+
+    <!-- Header cell (row 0): match the day-of-week title row appearance -->
+    <Style Selector="^:header">
+      <Setter Property="MinHeight" Value="38" />
+      <Setter Property="MaxHeight" Value="38" />
+      <Setter Property="Padding" Value="0" />
+      <Setter Property="FontStyle" Value="Normal" />
+      <Setter Property="HorizontalContentAlignment" Value="Center" />
+      <Setter Property="Foreground" Value="{DynamicResource CalendarViewForeground}" />
+      <Setter Property="FontWeight" Value="SemiBold" />
+    </Style>
+  </ControlTheme>
+
+</ResourceDictionary>
diff --git a/src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml b/src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml
index 0daa900a4c4..1cf2c755929 100644
--- a/src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml
+++ b/src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml
@@ -24,6 +24,7 @@
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/CalendarDayButton.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/CalendarItem.xaml" />
+                <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/CalendarWeekNumberLabel.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/Carousel.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/CheckBox.xaml" />
                 <MergeResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/FlyoutPresenter.xaml" />
diff --git a/src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml b/src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml
index 9eba0a413b2..88083598c71 100644
--- a/src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml
+++ b/src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml
@@ -128,7 +128,10 @@
                       FirstDayOfWeek="{TemplateBinding FirstDayOfWeek}"
                       IsTodayHighlighted="{TemplateBinding IsTodayHighlighted}"
                       SelectedDate="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedDate,
-                                                     Mode=TwoWay}" />
+                                                     Mode=TwoWay}"
+                      IsWeekNumberVisible="{TemplateBinding IsWeekNumberVisible}"
+                      WeekNumberRule="{TemplateBinding WeekNumberRule}"
+                      WeekNumberHeader="{TemplateBinding WeekNumberHeader}" />
           </Popup>
         </Grid>
       </ControlTemplate>
diff --git a/src/Avalonia.Themes.Simple/Controls/CalendarWeekNumberLabel.xaml b/src/Avalonia.Themes.Simple/Controls/CalendarWeekNumberLabel.xaml
new file mode 100644
index 00000000000..492664e6984
--- /dev/null
+++ b/src/Avalonia.Themes.Simple/Controls/CalendarWeekNumberLabel.xaml
@@ -0,0 +1,35 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+  <ControlTheme x:Key="{x:Type CalendarWeekNumberLabel}" TargetType="CalendarWeekNumberLabel">
+    <Setter Property="FontSize" Value="9.5" />
+    <Setter Property="FontStyle" Value="Italic" />
+    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundLowBrush}" />
+    <Setter Property="HorizontalContentAlignment" Value="Center" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Border BorderBrush="{DynamicResource ThemeBorderLowBrush}"
+                BorderThickness="0,0,1,0">
+          <ContentPresenter Name="PART_ContentPresenter"
+                            ContentTemplate="{TemplateBinding ContentTemplate}"
+                            Content="{TemplateBinding Content}"
+                            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                            Margin="{TemplateBinding Padding}" />
+        </Border>
+      </ControlTemplate>
+    </Setter>
+
+    <!-- Header cell (row 0): match the DayTitleTemplate appearance (Bold, normal foreground) -->
+    <Style Selector="^:header">
+      <Setter Property="FontStyle" Value="Normal" />
+      <Setter Property="FontWeight" Value="Bold" />
+      <Setter Property="Padding" Value="4,0" />
+      <Setter Property="HorizontalContentAlignment" Value="Center" />
+      <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}" />
+    </Style>
+  </ControlTheme>
+
+</ResourceDictionary>
diff --git a/src/Avalonia.Themes.Simple/Controls/SimpleControls.xaml b/src/Avalonia.Themes.Simple/Controls/SimpleControls.xaml
index ef6de7ca78e..a876ae3edf3 100644
--- a/src/Avalonia.Themes.Simple/Controls/SimpleControls.xaml
+++ b/src/Avalonia.Themes.Simple/Controls/SimpleControls.xaml
@@ -56,6 +56,7 @@
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/CalendarButton.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/CalendarDayButton.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/CalendarItem.xaml" />
+        <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/CalendarWeekNumberLabel.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/Calendar.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml" />
         <MergeResourceInclude Source="avares://Avalonia.Themes.Simple/Controls/Slider.xaml" />
diff --git a/tests/Avalonia.Controls.UnitTests/CalendarTests.cs b/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
index 75ba43c9e4f..e8073087f19 100644
--- a/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
@@ -440,5 +440,77 @@ public void CalendarItem_Should_Reset_YearView_Mouse_Down_Flag_On_Detach_From_Vi
             // Verify the flag was reset
             Assert.False((bool)field.GetValue(calendarItem)!);
         }
+
+        [Fact]
+        public void WeekNumberHeader_Defaults_To_Hash_Symbol()
+        {
+            var calendar = new Calendar();
+            Assert.Equal(calendar.WeekNumberHeader, "#");
+        }
+
+        [Fact]
+        public void WeekNumberHeader_Can_Be_Set_To_String()
+        {
+            var calendar = new Calendar();
+            calendar.WeekNumberHeader = "CW";
+            Assert.Equal("CW", calendar.WeekNumberHeader);
+        }
+
+        // --- Week number tests ---
+
+        [Fact]
+        public void IsWeekNumberVisible_Defaults_To_False()
+        {
+            var calendar = new Calendar();
+            Assert.False(calendar.IsWeekNumberVisible);
+        }
+
+        [Fact]
+        public void WeekNumberRule_Defaults_To_Culture_CalendarWeekRule()
+        {
+            var calendar = new Calendar();
+            Assert.IsType<CalendarWeekNumberRule>(calendar.WeekNumberRule);
+        }
+
+        [Fact]
+        public void IsWeekNumberVisible_Can_Be_Set()
+        {
+            var calendar = new Calendar();
+            calendar.IsWeekNumberVisible = true;
+            Assert.True(calendar.IsWeekNumberVisible);
+        }
+
+        [Fact]
+        public void WeekNumberRule_Can_Be_Set()
+        {
+            var calendar = new Calendar();
+            calendar.WeekNumberRule = CalendarWeekNumberRule.FirstFourDayWeek;
+            Assert.Equal(CalendarWeekNumberRule.FirstFourDayWeek, calendar.WeekNumberRule);
+        }
+
+        [Theory]
+        // ISO 8601: week 1 of 2023 starts on Monday 2 Jan 2023
+        [InlineData(2023, 1, 2, CalendarWeekNumberRule.FirstFourDayWeek, DayOfWeek.Monday, 1)]
+        // 2022-12-31 is still in ISO week 52 of 2022
+        [InlineData(2022, 12, 31, CalendarWeekNumberRule.FirstFourDayWeek, DayOfWeek.Monday, 52)]
+        // .NET bug: 2018-12-31 is a Monday and is ISO week 1 of 2019, not week 53 of 2018
+        [InlineData(2018, 12, 31, CalendarWeekNumberRule.FirstFourDayWeek, DayOfWeek.Monday, 1)]
+        // US rule: week 1 always starts on Jan 1
+        [InlineData(2023, 1, 1, CalendarWeekNumberRule.FirstDay, DayOfWeek.Sunday, 1)]
+        [InlineData(2023, 12, 31, CalendarWeekNumberRule.FirstDay, DayOfWeek.Sunday, 53)]
+        // Iso shorthand: same results as FirstFourDayWeek + Monday
+        [InlineData(2023, 1, 2, CalendarWeekNumberRule.Iso, DayOfWeek.Sunday, 1)]
+        [InlineData(2022, 12, 31, CalendarWeekNumberRule.Iso, DayOfWeek.Sunday, 52)]
+        [InlineData(2018, 12, 31, CalendarWeekNumberRule.Iso, DayOfWeek.Sunday, 1)]
+        public void GetWeekOfYear_Returns_Correct_Week_Number(
+            int year, int month, int day,
+            CalendarWeekNumberRule rule,
+            DayOfWeek firstDayOfWeek,
+            int expectedWeek)
+        {
+            var date = new DateTime(year, month, day);
+            int week = DateTimeHelper.GetWeekOfYear(date, rule, firstDayOfWeek);
+            Assert.Equal(expectedWeek, week);
+        }
     }
 }