The WEEKDAY ( ) Function

The WEEKDAY ( ) function returns a number that corresponds to the day of the week upon which a specified date falls.

Syntax:  

WEEKDAY(serial‑number [, return‑type] )

Serial‑Number:

The date (or a string representation of the date) you want to work with

Return Type:  

An integer that determines how the value returned by WEEKDAY ( )

corresponds to the days of the week:

1. The return values are 1 (Sunday) through 7 (Saturday); this is the default.

2. The return values are 1 (Monday) through 7 (Sunday).

3.The return values are 0 (Monday) through 6 (Sunday).

Example: 

If you want to see the day of Sep. 20, 2011, type this formula in any
cell =WEEKDAY("9/20/2011")

Returns:                            4