what are the busiest days at the parks?
Depends on the time of year and whether the schools are out and if it's a public holiday and if there's Extra Magic Hours that day and whether it's raining or not.
To be honest in September I wouldn't worry about it too much.
I could be a smartalec and say days ending in "y" but I agree - weekends and public holidays are the worst.
But I think you'll find all the parks are a bit less crowded at the moment, tourism is down generally.
Next trip - Bali! (March 2015)
MK is always crowded. However, if you get there when it opens, you can pretty much get everything done before the crowds descend.
DHS is going to be crowded on the days they show Fantasmic now. I plan to avoid like the plague.
No holidays in Sept except Labor Day which is Sept 7th. Next holiday is Columbus Day, Oct 12th.