Hi Patrick,
The earliest example I can think of of an AC station going all-Christmas that wasn't part of a stunt or format change is that of WRVF-FM (101.5 The River) in Toledo, Ohio, which has been going all-Christmas since 1995. That was the year that the station dropped its former WLQR-FM identity (it had been a Beautiful Music station under those calls until about 1987 or so) and became WRVF. "Christmas On The River" has been a tradition in Toledo since. There may have been other AC stations that did it even before WRVF, but if so I'm not aware.
I have heard that WOKY-AM in Milwaukee during its Adult Standards days was one of the first all-Christmas stations in the country, if not the first. Dial Global's America's Best Music (formerly WW1 AM Only) network has been going all-Christmas for several years now and since WOKY used to be an affiliate of that format, it is possible that this was based on WOKY's example. I'm not sure e
What Corky mentions re: all-Christmas formats really taking off in 2001, with the 9/11 tragedy taking place only months earlier, is my recollection as well. WOOD-FM 105.7 in Grand Rapids, MI dropped its longstanding "EZ105.7" identity over Thanksgiving Weekend of 2001 and relaunched as "Star 105.7" with all-Christmas, and I can remember one of the on-air promos they used to promote the change said, "Because we need a little Christmas now." As I recall, WRVF also started "Christmas On The River" earlier that year.
One of WOOD-FM's AC competitors, Soft AC/MOR hybrid WTRV (100.5 The River), was another early Christmas-music convert; as I recall, they started in 1999, or possibly '98. Ever since 2001 Grand Rapids has had at least two all-Christmas stations competing for the holiday music audience, though WTRV got out of the Christmas music battle last year and its sister station 95.7 WLHT went all-Christmas instead (as it did again this year). I say at least two stations because Grand Rapids also had a WW1 America's Best Music station (810 WMJH) for a time (now Spanish), which gave the market three choices for continuous holiday music.