Several customers contacted me about a problem they faced in their date table. They created a date table in Power BI , deriving the weeknumber from the date. Obviously, the solution is to use the best date table available, created by Melissa de Korte, an Enterprise DNA Expert. It is available on the portal in the Power Query Series.
For those who prefer a date table in Power BI, the solution is to use the following DAX function for weeknumber
WEEKNUM(Datetable[Date], 21) so although by default 1 or 2 show, type 21 and your weeknumber will be correct, January 4 = week 1.