refactor(datetime): rename abbreviated month and weekday constants to full names

[a-skua]

Summary

• Rename month constants from abbreviated to full names (e.g., JAN → JANUARY)
• Rename weekday constants from abbreviated to full names (e.g., SUN → SUNDAY)

Motivation

Addresses feedback from @timreichen in #6910 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.22%. Comparing base (c835e4d) to head (5557352).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6939   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files         584      584           
  Lines       43146    43146           
  Branches     6908     6908           
=======================================
+ Hits        40653    40656    +3     
+ Misses       2441     2439    -2     
+ Partials       52       51    -1     

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[kt3k]

LGTM

Note to other maintainers: These values haven't been published yet. So this change can be landed as 'refactor'