Oh I love going at holiday time. My fiance and son (10yrs) and I have gone for Thanksgiving in '07, '09 and just booked for this coming. It is so beautifully decorated I can't imagine going at any other time of year. The only thing I noticed that was considerable between the 1st and 2nd trips was the crowds depending on the dates we went. When we went in '07 we left Thanksgiving morning - Thurs. and stayed until the following Monday. We had a super early flight so we were able to spend most of Thanksgiving Day at Magic Kingdom and it was relatively quiet - and from Friday on things were way more busy crowd-wise. For last year's trip in '09 we flew out 2 days earlier on a Tuesday and stayed through the following Monday - and the pattern seemed to be the same in that from Thanksgiving day on it got more crowded. I have never been in the very beginning of Dec. so I don't know what it is like crowd-wise then.
For this year we are actually going from the 18th - the 26th, which will have us there 1 week prior to Thanksgiving and going home the day after. I wonder if it will be a little less busy that way.
As far as tips and advice, I would just recommend seeing the Osborne Family Spectacle of Dancing Lights if you can, it really is amazingly beautiful. The Mickey's Christmas Party is a lot of fun, the parade is really cool but get there early if you want to get a good viewing spot. The fireworks display there is really magical as is the "snow" that comes down on Main Street. The candlelight processional in Epcot is also really nice and we found that if you are walking through the park when it's going on you can still see and hear a good part of the choir etc. without having to pay for the seating.
Enjoy your trip, I'm sure you'll love it.
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