Christmas' origins are unrelated to Christianity. The date, the trees, the gifts, the eggnog, the caroling, the mistletoe and holly, the candles and lights, the yule log; pretty much all of it (except the manger scene) comes directly from European Pagan and Animist traditions. Christmas is (still) the celebration of the Solstice, the most significant annual astronomical event, harbinger of warmer weather and therefor survival itself, for both hunter-gatherers and farmers. The solstice is significant to every temperate-latitude indigenous culture globally, which is to say (almost) everyone's ancestors. With all this in mind, it is wholly appropriate for us Atheists to get into the "spirit" of Christmas--it is Applied Astronomy through-and-through, and celebrating it helped our ancestors survive long enough to make our births possible. Just ignore the manger scenes; they are recent newcomers to the holiday season.
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