Using the WEEKNUM function:
The arguments for the WEEKNUM function are:
|serial_number||Required||A date within the year. Dates should be entered by using the DATE function, or as results of other formulas or functions. For example, use WEEKNUM(2014,3,17) for the 17th day of March, 2014. Problems can occur if dates are entered as text.|
|return_type||Optional||A number that determines on which day the week begins. The default is 1.
• System 1 The week containing January 1 is the first week of the year, and is numbered week 1.
• System 2 The week containing the first Thursday of the year is the first week of the year, and is numbered as week 1. This system is the methodology specified in ISO 8601, which is commonly known as the European week numbering system.