The answer above is designed to work for dates.
The WEEKDAY formula looks at the date in the cell, calculates the day of the week to which it relates, and returns a value based on that day.
e.g. Monday = 1, Tuesday = 2, ... , Sunday = 7.
Thus if the date in the cell is 13/01/2007 (or 13-Jan-2007, or whichever date format you have chosen) it will calculate that it is a Saturday and return a value of 6.
In my example formula only return values above 5 change the format of the cell i.e. Saturday (6) and Sunday (7).