rocknbil - 4:41 pm on Jul 9, 2011 (gmt 0)
define an "origin point" which is some Monday midnight some time in the past.
I write a timesheet application years ago and this was the key element - to get specific week days of anything past or future, I calculated it as "this day is X time from the beginning of the Unix epoch" - then you can use mySQL date math to calculate any day in reference to this day. Ex:
(Monday of last week)
Today is Saturday and the current second is X seconds from the beginning of the epoch, Monday of last week is 12 days ago (Which requires a little calculating depending on what week start your country uses.)
Y = X - (12 * 24 * 60 * 60)
Should give you the exact same hour and second from Monday the week before on which you apply date formatting or whatever.