WEEKNUM(date,method)
date: date serial value
method: numbering system, defaults to 1
WEEKNUM calculates the week number according to method which defaults to 1.
If method is 1, then weeks start on Sundays and January 1 is in week 1. If method is 2, then weeks start on Mondays and January 1 is in week 1. If method is 150, then the ISO 8601 numbering is used.