There's two answers... one for listeners and one for salespeople and advertisers.
Listeners may want to wait till Thanksgiving Friday or Dec. 1. But AC stations, especially in markets where more than one station flips to Xmas music, want to be first. You want advertisers to start hearing the music in the store or office and realize they've got to buy time on the station NOW. You know the ratings go up for Xmas music. I've never heard a story where a station was HURT by playing Xmas music or playing it too early.
A few years ago, three stations in Philadelphia switched to Christmas music on or around Thanksgiving: the market's top AC (and often #1 overall) WBEB. The market's CBS Oldies station WOGL. (This was the year all CBS Oldies stations went all-Xmas in December except in Dallas where CBS also owns the big AC station.) And the market's upstart AC station, WNUW, which had just flipped from Smooth Jazz. How did they finish in the December ratings? They were all in the Top 3.
1)WBEB 2)WOGL 3)WNUW. It was WNUW's highest ratings ever. When they went back to regular AC music after the holidays, their ratings sank and they are now All-Sports WPEN-FM.
So when you hear an AC station switch to Bing Crosby and Nat King Cole before Thanksgiving, don't give them a hard time. They're doing it to be first and to get the advertisers on board.