Carolynleanne -
Early December is my preference. The crowds were very reasonable when I went during the first week of December last year.
If your decision is also largely based on the thorough, all-encompassing level of decorations that is so amazing at DLR, and you want to experience the full array of holiday offerings, December is a better month - or at least after Thanksgiving.
If you're a true Christmas time fan and you really enjoy beautiful trees and things like that, you must soak in the Grand Californian Hotel's tree and general festive lobby atmosphere, with Santa and Dickens carolers and all of that. Often times there is a guitarist strumming holiday favorites by the fire.
However, there have been reports that the GCH's tree doesn't go up until the day after Thanksgiving - I still can't quite figure out if that's the case every single year or just certain years. It seems like there have also been years when folks said the tree went up right before Thanksgiving.
I am also a big fan of the Paradise Pier Hotel lobby tree. Towering, majestic, and aglow in warm blue-green lights, the PPH tree is very striking. Just on a strictly color-based level, I think the PPH tree is the prettiest one. The GCH tree is elegant and gorgeous in its own way, and fits the best in its particular setting.
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Disneyland Hotel usually has a main tree in each tower, and some interesting decorations around the entrance. Goofy's Kitchen has a rather wacky, whimsical tree as well.
In any case, if these things are important to you to see, it's good to keep in mind that they may or may not be in place before Thanksgiving.
I've been at DLR over the CM party nights in December when DL closed early, but DCA stayed open. It was not bad at all. Last year my first day at DLR was on the final night of the Candlelight Processional and that was not even as bad (crowd-wise) as I expected it to be.
Whatever you decide, you'll have a wonderful time!