The Weekday function allows you to determine on what day of the week a date falls. This function returns a date value, with the default (leaving the argument blank) being: Sunday: 1, Monday: 2, Tuesday 3, Wednesday: 4, Thursday: 5, Friday:6, Saturday 7
The format of the Weekday function is: WEEKDAY(Serial Number, Return Type)
For instance if we used this function without an argument on the date 9/17/2012 which was located in Cell A1 (which is a Monday), the function would read:
WEEKDAY(A1) and the value returned would be: 2.
If you wanted the week to begin on Monday instead of Sunday, you would use the argument 2: WEEKDAY(a1,2), which would now return a value of 1. The screenshot below show the list of available arguments you can us.