So you want a 1 for every Mon thru friday and a zero for every sat and sunday.
There are two different methods for doing this. One is to create a date table and put in one row for every day and have a collumn with a value of 1 if its a work day or not. Then you relate on day to that table and sum this column.
Another shortcut is to do a case statement and sum it. I don't have a SQL server available to me to test it right now but here is the basic idea.
sum (case somedate when monday = 1
when tuesday = 1
when wed = 1
when thur = 1
when fri = 1
You can determine the mon-friday using date part. It's not the exact code but this should give you the idea.