See

http://cpearson.com/excel/weeknum.htm

In article ,
sam wrote:

using excel 2003 on xpSp2. i'm aware that MS says its normal that jan 1 is
week 1 when you're using the weeknum function. (in this year.only 1 day in
week1).. how does one modify this so that week 1 of the weeknum function is
the first 4 day week.. is there a mod i need to apply?