Function Calendar Wrong WeekNum

Anonymous , Try these column and see if last column give you desired output

Create a week start date

Week Start date = 'Date'[Date]+-1*WEEKDAY('Date'[Date],2)+1 //Monday

or

Week Start date = 'Date'[Date]+-1*WEEKDAY('Date'[Date],1)+1 //Sunday

Week Year = year([Week Start date])

First Week start of year = minx(filter('Date', [Week Year] =earlier([Week Year])),[Week Start date])

Week of Year = Quotient( datediff([First Week start of year],[Date],Day),7)+1

You should use WEEKNUM ( 'Date'[Date], 21 )

Returntype 21 = ISO 8601 Week or the European weeknumbering system, no need for custom formulas or functions.

You could also read more about the Weeknum function here: WEEKNUM – DAX Guide