When is Christmas not Christmas? When its equivalent appears under another name in a holiday episode of a TV series or movie franchise. TV Tropes has a page on this phenomenon:You Mean Xmas
It's not unusual for TV series to have "Christmas" episodes even if they're set in a time or place where Christmas doesn't exist. An episode of XENA, WARRIOR PRINCESS featured "A Solstice Carol." MY LITTLE PONY: FRIENDSHIP IS MAGIC has "A Hearth's Warming Tale," set on the holiday celebrating the occasion when the three types of ponies worked together to save the fledgling realm of Equestria from the terrible Windigos. (This story combines elements of A CHRISTMAS CAROL and IT'S A WONDERFUL LIFE.) Then there's the infamous STAR WARS holiday special, set on the Wookie home planet at the season of Life Day. (I've never seen this film, so all I know is what's summarized on TV Tropes; it has never been re-aired, because it's so abysmal that Lucas himself loathes it.) The inhabitants of Fraggle Rock celebrate the Festival of the Bell in "The Bells of Fraggle Rock," at the time of year when the Rock slows down and would freeze forever if the Fraggles didn't ring their bells to awaken the Great Bell. The characters in DINOSAURS have Refrigerator Day, appropriately commemorated by lavish feasting. Although BEAUTY AND THE BEAST takes place in the world as we know it, members of the secret underground community where Vincent (the Beast) dwells celebrate "Winterfest" instead of Christmas. Print fiction features a similar phenomenon. There's a Midwinter Festival in Mercedes Lackey's Valdemar universe. The people of Discworld have Hogswatchnight, as portrayed in detail in Terry Pratchett's HOGFATHER. The world of Steven Universe is an exception to this pattern. Its canon establishes that the invasion of the alien Gems thousands of years ago altered Earth so radically that Christianity doesn't exist, so there's no Christmas, Easter, Valentine's Day, etc. However, virtually every temperate-zone culture in the world has a winter solstice celebration with such elements as feasting, lights, greenery, and bells, so it seems likely that the people in this series would have one, too. If they do, apparently the producers and writers simply haven't considered it necessary to mention.
In the animated special ARTHUR'S PERFECT CHRISTMAS, Arthur's bunny friend gets so stressed out by his divorced mother's frantic attempt to make Christmas perfect that he wants to invent their own family holiday instead, "Baxter Day." An episode of SEINFELD popularized the anti-Christmas holiday of Festivus, which includes the Airing of Grievances (when everybody complains to everybody else about offenses committed through the year) and an aluminum pole instead of a tree. In short, the human spirit seems to crave festivity at the dark of the year.
A satirical essay by C. S. Lewis imagines what the ancient Greek historian Herodotus would have made of the modern British Christmas. Herodotus concludes that Exmas and Crissmas can't possibly be the same holiday, because even barbarians wouldn't go through all that expense and bother for a god they don't believe in:Xmas and Christmas
Margaret L. CarterCarter's Crypt