We recently returned from WDW. For the first time, we booked the Backstage Magic Tour. One stop on the 7-hour tour was at the Holiday Department. It was interesting to see how the trees and other decorations are stored and maintained. The work continues all year long to clean, repair, replace, and then store the items for the next year.
We were told that the decorating begins around November 1st or 2nd and each individual resort or park takes only 1 night to decorate, except, I believe, MK, which may take 2. Late each night the crews show up at selected sites and finish by morning. The crews then move on the next night to other sites. It must seem like Santa's elves have visited when resort guests wake up the following morning to find their resorts transformed!