the decorations start getting put up in early November and completed before Thanksgiving. They are taken down starting the second week of January.
There is a permenant staff of 25 people working on Christmas decorations year round. During November and January, there is an additional 1,000 people to put up/take down the decorations. Work is generally done during the night.
They are working on next year's decorations already. New trees for the YC & BC have arrived and will be Disneyized.
Decorations are even ready for POP Century if/when it opens.
At the WL, people stay up and camp out on the balcony to watch them put the tree and other decorations up. Did you know that the WL was the first Disney resort to be designed with Christmas in mind. Where the tree was to be located, there was extra support, electrical outlets, etc. The tree actually sits in a pit in the floor where the supports, electrical outlets, etc are located.
Please, someone stop me before I really start babbling

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You too can learn these and other more interesting facts during the Yuletide backstage tour.
Ed