WORKDAY
Category: DateTime Function.
Description: Returns a date that is a specified number of working days (excluding weekends and holidays) before or after a start date.
Syntax: WORKDAY(start_date, days, holidays)
All weekdays are counted; weekends and dates identified as holidays are excluded.
To specify holidays to exclude, use an optional third argument that is a range of cells containing dates or an array of serial numbers representing dates.