Last year (2001) we left the Tuesday after Thanksgiving, which I believe was 12/1...it was absolutely the best time of year that we ever traveled. No lines at the airport, weather was great. I would by far recommend it. If possible try to leave either a Tuesday or Wednesday, not as many people traveling and the rates seem to be a bit cheaper. I think if you left that Sat, Sun Or Monday after Thanksgiving it would be a lot heavier travel day(probably more expensive too). We always leave on a Wednesday for WDW, that way we don't feel the pressure of taking advantage of the later nights and parades which is especially important on the off season (they are usually only offered on weekend nites), & it is much less expensive to fly. I think it is much more relaxing this way.
