Winter/spring/summer/fall celebrations are human in origin. Certainly some of the specific tropes of current Holidays are copied or held over from previous practices, that's obvious but not sinister. For Christmas, those happen to be passed on from the previous religion in the location in which Winter celebrations were more important due to the long nights. Specifically Northern Europe, which just happened to be where Christianity spread at the right historical time. But they were most likely held over by those converted Christians because they are nice rituals to have. They became so famous world wide, not because they were significant in any way, but because they were serendipitously carried to the Americas, and then US culture became world dominant for so long. Unless someone really thinks that people were willing to give up their ancestral religion, one which served their social needs perfectly, simply because this new upstart religion let them have a tree decorated on the Solstice. I can just see a cabal of Priests rubbing their hands and chortling, "Then, we'll let them hang up mistletoe and they will forget all about Baldur and become Christians!!!" That's contrary to human nature. They converted first, then carried the old familiar rituals with them. And I know that some were "forced conversions" (as reprehensible and stupid as that seems), but that makes the holding over even more obvious. Every Christmas for the last 40 years someone trots out this "Origins of Christmas" story as if it were repressed knowledge that was finally made known through the work of heroic Pagan holdouts, but my childhood Norwegian Lutheran church celebrated these origins and even though many are uncaring about the source, they don't deny it. Even I just deny the sinister mind-control conspiracy implications offered with them. If these celebrations were not already part of the extant culture of those new Christians, similar celebrations would have been invented spontaneously, because Winter Celebrations have pretty obvious markers. I mean, come on, burning a special log on the darkest, longest night of the year when every one in the village gathers together? Who wouldn't have thought of that? What I never hear mentioned is that those Pagan rituals were probably carried over from the previous local religions. Just as Christmas memes will be (have been) carried over into some of the new religious and secular Winter celebrations. The Japanese have integrated Santa Claus into their Shinto/Buddhist Winter celebration and even Festivus carries clear ancestors of a non-pagan, Americanized Christmas.