As bscs said usually Fri-Sun are the busier days of the week. When schools in session, to me Fridays are the worst, due to all the school field trips to the park especially this time of the year when school is almost out.
You can pretty much get a feel for which times of the year are slower by the time the park closes, usually if it closes at 6 or 7 PM then that time of the year is slower and the crowds are usually alot lighter if there are no special events (which usually are on the weekend too.)
Also on the days the park closes earlier, there are usually less shows offered too.