Crowds are no longer super low in September for a couple of reasons:
1. There's often a Run Disney event on a weekend in September.
2. In late August, the So Cal Select AP block outs end. When that happens, all of the locals who have those APs head to DL in DROVES! Evenings especially can be pretty crowded, especially on a Friday evening. And once those So Cal AP block outs end, every Sunday will be really busy. REALLY busy. I have friends who live 30 min to an hour's drive away and they often report this at that time of year.
The last day of the So Cal Select AP is on Sunday 8/19. So Cal Select APs are NOT blocked out on:
Mon 8/20 - Thurs 8/23
Mon 8/27 - Thurs 8/30
"Legacy" AP like the regular So Cal AP (which are not available for sale, but some AP holders still have them and they're still active APs) are NOT blocked out in Aug or Sept on the following dates:
Mon 8/20 - Fri 8/24
Sun 8/26 - Fri 8/31
Sun - Fri in all of September except for Labor Day weekend
Then in September, they're NOT blocked out pretty much Mon-Fri for the whole month, with the exception of Labor Day.
So if you DO go in August and if you DO go between 8/20-8/30, be prepared on those Mon-Thurs week days for the parks to be MOBBED in the late afternoon to early evenings. It's often around 8/25-8/26 or so that they take Haunted Mansion off line in order to put up the Nightmare Before Christmas overlay. So all of those AP holders who want to go on HM and probably Space Mountain, too, will go to the parks JUST to go on 1 or 2 rides.
I think that you should go in the first half of August. Or at least go BEFORE the So Cal Select & So Cal APs block outs end. So finish your trip before Monday 8/20. Why? For a couple of reasons:
- Pretty much all of Arizona and I believe a lot of Utah and Nevada kids are back in school in early August. Disneyland gets a lot of tourists from Arizona, Utah, and Nevada.
- Some So Cal school districts are also back in school in early August. For example, LA Unified goes back to school on 8/13.
- This leaves you with families being at Disneyland whose kids go back to school in late August or right after Labor Day. OR they're families with young children who aren't in school yet OR they're families who home school.
- The first half of August (through 8/19) will STILL have daily fireworks and the full schedule of summer entertainment.
- During the last couple of summers, many people here have reported that Saturdays in the summer were actually LESS crowded and MORE pleasant than a week day in September or October. Crazy, right?
- A big part of Disneyland's regular visitors are locals from southern California. And because if you are a southern California resident, you can get an AP and pay it on a monthly basis, it has made it a lot more affordable, thus resulting in a lot of So Cal residents getting APs over the past few years.