To get the week number of a given date, i.e. to find out where the week falls numerically within a year you use the WEEKNUM function.
serial_num : is a valid date. E.g. August 15, 2008 or 12/10/2008.
Return_type : is a number that determines on which day the week begins. (1 = Sunday, 2 = Monday). The default is 1.
If cell A1 = 10/19/2008 , which is a Sunday, then:
=WEEKNUM(A1,1) will return a week number of 43 (week begins in Sunday).
=WEEKNUM(A1,2) will return a week number of 42 (week begins in Monday).
WEEKNUM function is part of the Analysis ToolPak add-in. If it is not available in your list of functions then you have to install the Analysis ToolPak. Check the following link for more help:
Important note (from Microsoft Excel Help)
The WEEKNUM function considers the week containing January 1 to be the first week of the year. However, there is a European standard that defines the first week as the one with the majority of days (four or more) falling in the new year. This means that for years in which there are three days or less in the first week of January, the WEEKNUM function returns week numbers that are incorrect according to the European standard.
Applies to Excel 2003