Our experiences have always been that they start at Thanksgiving and are done by the Sunday after it. I don't recall seeing any schedule in print anywhere.
I have always been told they have a schedule to have everything up by Sunday after Thanksgiving. Disney has an entire staff that only works on the Christmas decorations. I usually try to go the first or second week in December and everything is always full scale up and running.
I had seen a schedule two years ago, when we visited at Thanksgiving, indicating as to when the decorations at each of resorts would be completed. I hope they put the same type of schedule out this year as we are planning a visit again over the holiday.