Working out the number of Mondays in a month isn't too hard. Working out how many fall between two dates is trickier. The Mr Excel video series offers two ways to get that task done automatically with a formula.
This isn't the kind of calculation problem you'll encounter regularly, but it's tricky enough that it's useful to have someone else do the heavy lifting. You can, of course, easily adjust either formula to handle days of the week other than Monday; check out the video for the details. For more Excel trickery, try using it to solve Sudoku.