Shane's formula works if your DAYs are text values or date values formatted
using the TEXT function. If, instead, your DAYs are actual date values
displayed using custom formats, then a formula like this should work:
=IF(OR(WEEKDAY(B1)=1,WEEKDAY(B1)=7),0,7.5)
This assumes that the first day is in cell B1.
Hope this helps,
Hutch