Hi,
Either of these formulae will do it, where C3 contains the date in question (eg. 27 June 2011):
Code:
=NETWORKDAYS(EOMONTH(C3,-1)+1,C3)
=NETWORKDAYS(DATE(YEAR(C3),MONTH(C3),1),C3)
If you are using Excel 2003 or earlier then the Analysis Toolpak must be installed.
These formulae only take weekends into account. If you want bank holidays to be included then you can - you'll need to define a range where you keep a list of bank holidays and then pass a reference to it into the 3rd, optional parameter of the NETWORKDAYS function.