ISOWEEKNUM(date)
date: date serial value
ISOWEEKNUM calculates the week number according to the ISO 8601 standard. Weeks start on Mondays and week 1 contains the first Thursday of the year.
January 1 of a year is sometimes in week 52 or 53 of the previous year. Similarly, December 31 is sometimes in week 1 of the following year.