I programmed Christmas music for a #1 AC in my market for years. New Christmas music wasn't even considered unless there was something very unusual about it, with a strong buzz, etc. The format exists basically with traditional and more contemporary titles. And by contemporary I don't mean "new", I mean anything from the 80's, 90's, and beyond. Even Elton's "Step Into Christmas" was considered contemporary! At my station we were very careful to keep the two apart by atleast two songs. Pretty simple? No, not really. The flow was terribly difficult to measure, especially when transitioning. But the clear answer was good imaging, a bit longer than non-Christmas imaging to deflect the major change in tempo and feel. And there was always that religious traditional song that might get scheduled after "Grandma got run over"...a quick eye check of Selector was indeed important!
All that said, I never really understood how Christmas music could be played before Thanksgiving. But, I am not the audience. I have since learned there is a passionate audience for this, and that is why it almost always wins. Not only on radio, but has anyone checked out cable tv's Hallmark Channel? They get started in July! Then go all in by October. If this didn't work for them, they wouldn't do it. Same with radio.