When all the decorations go up is variable...one year we were staying at the Yacht Club for T-giving and I got up super early to go for a run and got to see the CMs putting up the Yacht Club decorations, both outside and in the lobby...another year, also during an early-early morning run, I left SSR (the Fri after T-giving) w/all "normal" at around 5 and when I got back the lobby was decorated...
Other years, we've arrived for our T-giving week (usually the Mon or Tues prior to T-giving) to a fully-decorated resort...
Pretty sure the "target" is to have everything up by the "start" (sunrise?) of the Fri after T-giving, but how much earlier than that it occurs varies...generally, parks get decorated before resorts, tho, and parks w/special holiday events (like the Osbourne lights) get "dressed up" earlier, too...