Historically, the lowest crowds are January (after New Year's weekend), September and the first 2 weeks in December. The big difference is weather - September is still quite hot, and wet (it's still Hurricane season). January and December are VERY unpredictable - you can have highs in the 70s for a week and then suddenly it will plummet to 50 degrees with overnight lows in the 30s. But there's very little rain that time of year. October is a nice time to go, but it's become more crowded in recent years because it has the reputation of low crowds and perfect weather (plus that's now "convention season" in Orlando). November can be good, too, if you avoid Thanksgiving and the New Jersey teacher's holiday/super soap weekend.