All ISO-weeks have 7 days and begin on a Monday. Week 1 begins on the monday of the week in which the first Thursday of the year falls. This means that week 1 can start in the year before, and week 52 or 53 can end in the year after!
We use a formula to calculate the weeknumber of the date listed in field A1:
=INT((A1-DATE(YEAR(A1-WEEKDAY(A1-1)+4);1;3)+WEEKDAY(DATE(YEAR(A1-WEEKDAY(A1-1)+4);1;3))+5)/7)