It would definitely be the first week of November. We've been twice in November, and the weather was just sublime ... temps in the mid to high 70s; little to no humidity. And little to no crowds, either. I'd choose the first week so as not to have to see Christmas decorations up yet; I like seeing Disney in its original state.
I agree with PP. I love the last week Oct / 1st week Nov.
I am a fan of the F & W Fest over at Epcot, so that is part of the driving force. The weather that time of year is absolutely perfect. Your in the 70s all day with lots of sunshine and just cool enough at night to be comfortable. The crowd levels are lower that time of year too so you can see and do more not only at Disney but the surrounding Orlando area.
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