As suggested, do a search on this forum... there's a particullarly good and nasty fast function 
to calculate Work Days (week days, actually) at http://www.sqlservercentral.com/columnists/jmoden/calculatingworkdays.asp
Also as suggested, you will need to incorporate a "Holiday Table"... you can find out how to do that (and more with calendar tables and the like) at
Adam Mechanic was the guy who turned me on to Tally or Numbers tables. Haven't needed a full blown calendar table but have made a holidy table. It's definitely worth the read... please ignore the fact that he used an (ungh! 
) cursor for one thing... he must'a been feeling poorly that day 
--Jeff Moden
RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
Change is inevitable... Change for the better is not.
Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)