The Trick or Treat Party will not be in DCA this year. It's going to be held in
Disneyland, on every Tuesday, Friday and Sunday in October. So Disneyland will close hours early on those TOTP nights to everyone but the folks who are attending the TOTP. Of course, the TOTP costs extra. So if you don't want to attend the TOTP, you can head over to DCA where the new World of Color show will be. That may affect crowds in both parks a great deal - we won't know until WoC debuts and people start reporting back about crowd levels.
In any case, if Halloweentime begins on September 24th, as I suspect it will, and you decide to go then, you will miss the TOTP, which won't start until October 1st, but you will see all the other stuff like the fireworks, Haunted Mansion Holiday, the Halloween Round-Up, Space Mountain Ghost Galaxy and any other Halloween entertainment, food, merchandise, etc.
Suzy021372 is right - it was very crowded last year in October on the day we went as well. More so than it had been the previous year. It definitely got much worse at night. And this year, WoC may bring even more people to both parks. So unless you have a need to see the TOTP in October, the crowds might be slightly better in late September, I think.
Weather-wise, it is sometimes very, very hot in September. Sometimes it's raining. More often than not it will be quite warm, but October can be that way too (in 2008, we went during a heat wave in October, where it was in the upper 90's

!. September is often hotter than August or July!!