if you have a list of dates and want to find the first day of a particular month eg date in cell A1 is 25-Feb-2008 use =DATE(YEAR(A1),MONTH(A1),1) in cell B1 (this will give you the first day of a particular month) and to get the number of workdays between A and B use =ABS(NETWORKDAYS(A1,B1)) in cell C1.

will look something like the below

13-Feb-09 01-Feb-09 10