I have recently written a simple formula to achieve this same result but for finding the next Sunday following a date. It is as follows:
D18 is obviously the source of the random date. When using the WEEKDAY function, Excel resolves the date to an integer that represents the days of the week. Sunday is 1, Monday is 2, etc.
So, this basically says, If the date is a Sunday, than make the date the same, else take Excels numeric value for that date and add it to (8 - (the weekday value of that date).
Hope this gets you started... You will have to subtract from your date of course to get the Monday beginning that week! Good Luck!