IASW usually goes down for holiday overlay right after Halloween. It was down until Veterans Day last year. We were there until the 9th (2 days before Veterans day) and missed the opening by a day.
We've been there all of those months. I liked them all.

There are advantages to each I think. Sept. is low crowds (but can be HOT), Oct. is better as far as weather usually, but crowds pick up a bit because a lot of the year round schools get a fall break in Oct. Nov. is perfect (IMO) as far as weather and the crowds are low with the exception of around Thanksgiving. If you are there the first week or so of Nov. you get to see the holiday decorations going up a little each day.

Dec. is nice weather wise, crowds are low in the early part of the month but is insanely busy from around Xmas until after the new year.
I guess if I had to pick I'd probably chose November. There is something so magical about the parks when they are decorated for the holidays. We're personally contemplating going the second week of Nov, right after Veterans Day. The parks are usually pretty empty then, all the holiday overlays are up and the weather is generally beautiful. The only thing that's keeping me from yet is my daughter is starting kindergarten this year and I'm feeling guilty about pulling her out.