I noticed something a long time ago, but did not take time to investigate.
Maybe you can enlighten me ?
According to this post:
The expression WEEKNUM() returns the week number, 1 to 53 if parameter is a valid date.
Today, 21st may of 2021, is calculated as week number 21.
If I pick en english-formatted calendar (from : https://www.calendar-365.com/week-number.html), I get week number 20.
My own personal calendar displays number 20 as well.
I cannot explain why this happens, plus my locale is on France/French/Europe, both on Table locale and Sheets locale.
I notice the same output with WEEKNUM() performed in Sheets.
Any explanation welcome