We were in Magic Kingdom on September 27th. We made the mistake of going on a day that was 9am-10pm. Maybe "mistake" isn't the word so much, but it was sandwiched between two days of 9am-6pm where Mickey's Halloween party was happening. So my guess is people were all thinking the same thing and go a day that is 4 hours longer. So it was very busy on the day we went. We still did pretty much all of the core of the park, but we didn't get on everything. However, someone at our resort went the next day and said things were pretty much walk-ons. So maybe the key is going to MK the day of the Halloween parties.
Either way, we picked September thinking it was a slower time of year. Maybe it still is, I don't know, because Universal was fine crowd-wise both days we were there. Normal. Maybe we just picked a busy day for MK.