Been there in early January twice. Year I arrived on the 2nd, everything was decorated, but like PP, resort decorations were down by the third or fourth. I think my last park day, the sixth, the MK was all un-decorated, but a few items left to purchase.
Year I arrived on sixth or seventh, I don't remember any holiday decorations at all.
So, if you really want to experience Disney Christmas magic, go after Thanksgiving, but before Dec 20th. Weather is totally unpredictable, but crowds are low. I can't quite decide if its then or October that's my favorite, but its a great time to visit. I've had beautiful weather and below freezing weather then, but if you take a good variety of clothes, and check the forecast each morning before you leave your room, it's all reasonable weather.
Been in May, too. You'll most likely get some hot days there and crowds are heavier. It worked for the family unit traveling this last time, but May was too busy for me. We got frustrated by the crowds since we are normally there when crowds are lower.