I LOVED our September trip which was right around the first day Halloween decor went up. It was markedly busier on that Friday compared to the day before because APs came out to check out the new stuff. I would totally try to time it similarly again in the future, with a day or so before Halloween decor goes up and a couple days afterward.
Pros:
- most tourists' kids have school so you're really only dealing with AP crowds during the mid-day (so it's nice and empty early morning)
- MHP haven't started yet so you don't get kicked out of DL early
- weather is warm but not super hot
Cons:
- POTC, Blue Bayou, and Screamin were down for refurbishment
- fewer showtimes as it's the off-season
Hope this helps!