NETWORKDAYS
Category: DateTime Function.
Description: Calculates the number of work days between two dates.
Syntax: NETWORKDAYS(start_date,end_date,holidays)
Dates should be supplied as cell references or as results of the DATE or other function that returns a sequential serial number representing a particular date.
All weekdays are included; weekends and dates identified as holidays are excluded, and not counted as working days.
To specify the holidays to be excluded, use an optional third argument that is a range of cells containing dates or an array of serial numbers representing dates.