“Mostly?” Sure. That’s a pretty broad term, and it was common back in the olden days of the 20th century to begin playing right around Thanksgiving and not full time. I remember coming out of a remote at some nightclub on the god-forsaken night before Thanksgiving (the crowd lived up to the reputation of it being one of the busiest party nights of the year and was especially obnoxious 😉) to hear a holiday tune at 2 or 3 or whatever hour of the morning it was).
Anyone from the AC to the CHR in town would play holiday songs sprinkled amidst the usual format. You can still find some of that, with stations what drop in the occasional holiday tune, while someone (and maybe more than one someone) is doing the all-holiday thing.
For stations that did some variation of the 24 or 36 hours of non-stop Christmas music, that stood out a bit more because it may have been the only time they were all-Christmas.
But this is long-covered ground. The sales more than justify the way things are now.