Not to be a jerk... but have you tried typing =workday(your arguments
here) in a cell? This is from Help:

Returns a number that represents a date that is the indicated number of
working days before or after a date (the starting date). Working days
exclude weekends and any dates identified as holidays. Use WORKDAY to
exclude weekends or holidays when you calculate invoice due dates,
expected delivery times, or the number of days of work performed.

If this function is not available, and returns the #NAME? error,
install and load the Analysis ToolPak add-in.

How?

On the Tools menu, click Add-Ins.
In the Add-Ins available list, select the Analysis ToolPak box, and
then click OK.
If necessary, follow the instructions in the setup program.
Syntax

WORKDAY(start_date,days,holidays)