Christmas decorations tend to go up by mid-November. We went Nov. 2012 and the decorations in the lobby of BCV were put in beginning the night of Nov. 16 and completed by Nov. 17th. Other park decorations were already up before then, though.
I'd recommend mid-November through the day before Thanksgiving crowds arrive (typically that Tues/Wed prior to Thanksgiving Thursday). They will begin having Mickey's Very Merry Christmas Party (MVMCP) events by then as well as the Osborne Lights at DHS. Candlelight Processional at EPCOT might even begin prior to Thanksgiving, but I cannot remember for sure.
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