The Networkdays() function should work for you. It is, however, an Analysis Toolpak function and therefore the Analysis Toolpak must be installed.

Go to Tools|Options|Addins and select the Analysis Toolpak...

example formula: =Networkdays(A1,B1,X1:X10)

Where A1 is Start date, B1 is End Date and X1:X10 contains holidays to ignore.