It depends how you define a week. I myself use the formula
=1+INT((A2-DATE(YEAR(A2+4-WEEKDAY(A2+6)),1,5)+WEEKDAY(DATE(YEAR(A2+4-WEEKDAY(A2+6)),1,3)))/7)
where A2 contains a date. The first week of year is the one with first
Thursday in it, and all weeks are 7 days long (ISO week).