Unless they change it, MMs are on the S and T days. Sat Sun Tues and Thurs.
This generally only changes for Grad Nights, which of course aren't held in September...also for the occasional other sort of private party, but I think it's safe to figure on S and T.
Remember, the parks are VERY close to each other, and most of us here won't give a whole day to one park. Well, my family sometimes does (whole days at DCA, back and forth on the other days), but most others here just hop back and forth!