WEEKDAY()
returns 0 to 6 depending on which day of the week it is. You could put logic in the code to ignore weekend conditions, and go back to the Friday.
Returns the weekday index for date (0 = Monday, 1 = Tuesday, … 6 = Sunday).
So if WEEKDAY(date) - 1 == 5 || WEEKDAY(date) - 1 == 6
then make it equal 4 (Friday) instead.
http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_weekday