• This is not a feature request (existing functionality does not work, not missing functionality).
  I will request features on forum or via support.
• I've searched and did not find a similar issue.

Describe the bug
ISO week numbers in the calendar month view are only displayed for users who have Monday set as their first day of the week. Users with
Sunday or Saturday as their week start preference cannot see week numbers, even though this is a useful feature for scheduling and
coordination.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Settings' → 'Appearance' → set 'Week start' to Sunday
2. Open Calendar app and view month view
3. Observe that week numbers are not displayed on the left side of each week
4. Change 'Week start' to Monday
5. Week numbers now appear

Expected behavior
ISO week numbers should be displayed for all users regardless of their week start preference. ISO 8601 week numbers are always Monday-based
by standard, so showing them doesn't conflict with different week start preferences. Other calendar applications (Google Calendar, Outlook)
display week numbers for all users.

Screenshots
(Optional: If you have screenshots showing the difference, add them here)

Desktop (please complete the following information):

• OS: macOS
• Desktop Environment, if applicable (on Linux): N/A
• Version: 3.119.3 (or your version)

Additional context

Current Code:
The restriction is in src/calendar-app/calendar/view/CalendarMonthView.ts around line 333-336:

weekDayNumber === 0 && attrs.startOfTheWeek === WeekStart.MONDAY

This condition prevents week numbers from displaying for Sunday/Saturday start users.

Proposed Fix:
Remove the && attrs.startOfTheWeek === WeekStart.MONDAY condition. ISO week numbers should be visible to all users as they represent a
standardized way to reference weeks, independent of visual display preference.

Use Cases:

• Users in North America often prefer Sunday as week start but still need week numbers for work
• International coordination requires standardized week references
• Project management and scheduling tools use ISO week numbers

I have a working fix:
I've implemented the fix by removing the Monday-only restriction. The change is minimal (2 lines) and maintains ISO 8601 compliance while
improving usability for all users.

Related:
I've also posted this as an enhancement suggestion on Reddit as requested: https://www.reddit.com/r/tutanota/comments/1o4ktqx/enhancement_show_iso_week_numbers_for_all_users/