Last year Aug 29 - Sept 10 -- the two weeks surrounding Labor Day Weekend.
1st week was most definitely more crowded than 2nd week.
We spent Friday before labor day at MK. It was more crowded than the prior Wednesday, but still easy. We spent Thursday before labor day and then Saturday of labor day weekend at AK. The difference in the crowd was shocking. You NEEDED FP on Saturday. The walkways were difficult to manuver. We had dinner at Rain Forest then hopped to MK for the parade. We got there around 730pm. I stood in a seriously 20 minute FP kiosk line only to find that there was nothing left that we were interested in. (no mountains, HM, JC, Buzz, PP, Ariel) So we just sat down for the parade and people watched because we were too tired to bother with lines. Despite how crowded it seemed, we could have walked up at 840 in frontierland and sat down with lots of places to choose. The park closed at 12am with extra hours until 2am. It was strange. It felt like people were everywhere, but doing NOTHING. Most rides had very little wait. We did splash mountain over and over, lots of Buzz. At 12:00am we left Tomorrowland and headed to just see what the Anna & Elsa line was like - -we did have FP for later in the week, but the posted time was 20 minutes, the CM said 10 minutes and she was right. 10 minutes to see Anna & Elsa! We jumped into the Cinderella and Rapunzel line and that actually took longer - -15-20 minutes. Still, less than 1/2 hour for us to see all 4 was awesome.
Those hours are not posted for this September, but I sure hope it changes.
I can't answer about Sunday as we used that as a rest day and did laundry & DTD - -which was too crowded for me. Monday we went to Typhoon lagoon and had a pretty easy day -- kids went on everything lots and lots of times. I don't think any line was over 10 minutes.
The week after labor weekend was noticeably less crowded that the week before.
One thing to note, all of my trips since 2008 have been in September, so I can only compare the crowds to other Septembers, not to an actually crowded time of year. I can say that Disney in September is more crowded now that when I first started my September trips.