Although there is no hard and set rule on when holiday decorations appear here are a few guidelines...
Decorations generally are completed at Magic Kingdom, Disney's Hollywood Studios and Disney's Animal Kingdom Park in mid-november in time for "Mickey's Very Merry Christmas Party" and other holiday events at these three Theme Parks.
Decorations generally are not completed until Thanksgiving day at Epcot, Downtown Disney, Typhoon Lagoon, Blizzard Beach, Disney's Wide World of Sports, and the Disney Resort Hotels. Epcot's "Holidays Around the World" generally begins the Friday after Thanksgiving and ends December 30th or 31st depending on the year.
Note, each year one of the water parks closes for it's annual refurbishment in Nov/Dec followed by the second park in Jan/Feb. The water parks alternate each year to allow a different park to be decorated each holiday season.
Decorations will begin to be removed on the first Sunday following New Years Day. The "The Osborne Family Spectacle of Dancing Lights" also generally ends on this first Sunday of the year.