I did some research into this a few weeks back. From what I can make out, the decorations at the parks and resorts are usually up to coincide with the first of the MVMCPs (which is 10th November this year). The Osborne Lights will probably start on the same day, as will the Jingle Jungle Parade at AK and Santa Goofy's Holiday Village at Camp Minnie-Mickey. Epcot's "Holidays Around the World" didn't start until 23rd November last year.