December.....
After venturing down year after year in May, last year we decided to take our son (11 months) in December for a quick little 4 night trip, it was great!
The weather was good, the parks weren't that crowded, plus if you go before Christmas, it's normall considered value season, they offer some nice rates at various on-site properties.
The holiday fun starts right around Thanksgiving so the window of opportunity to see the various decorations and to enjoy the holiday atmosphere is a good one!
Mickey's Very Merry Christmas Party is a pretty nice take.
We're heading back down in a couple of weeks, but our AP expires the 12th of December and we may try to squeeze an late November/early December quick trip. It's well worth it.
I hope this helps.
Take care
Bob