Windows11 Why is week number missing from the taskbar calendar

There's no setting for it - the taskbar calendar has never supported week numbers, and the Region date format workaround people suggest doesn't work either, since there's no week-number variable in the custom date format strings.

So it's third-party or nothing. Disclosure: I develop one of the options, ZefirCalendar, a tray calendar that shows ISO week numbers - https://apps.microsoft.com/detail/9P95DT1HKT15

Worth voting for it in Feedback Hub too. It's a small ask that keeps getting ignored because nobody files it.