The "usually accepted" way it was before stations had to try to a) get in the pocket of local retailers FIRST for Christmas and b) Cause more "real" announcers to want to commit suicide was...
Thanksgiving and fist week after: 1 per hour, Next week 2 per hour, 3rd week 3 per hour, 4th or week before christmas 4 or more Christmas per hour, some regular still played, then some kind of Christmas "special lineup" on the eve, and the day.
The DISadvantage to 24/7 Christmas WHEVENER and WHEREVER implemented early is that there's nothing SPECIAL to give your listeners or to "sell" FOR Christmas! We always have had a special program, and either our locally produced "A Christmas Carol" or the 1939, plus other special programs which we believe make the holiday fun to have in the background of family events. To plant Christmas JUST to get into retail stores PA systems early is a sure-fire cheap-ass way for a program director to not have to work for awhile, and to program during the "layoff" season in radio with people not thinking as much about who they're missing. I'm sure, given the big company endorsements of A-I that we won't need some announcers this year at Christmas, sad....because it's the time a good radio station MOST connects to its listeners in so many ways.