For instance, in that story, bedecking the hall with boughs of holly and ivy is kinda important to the plot. Not Christmas trees. We can know for a fact that Christmas trees were modern because account books record what sorts of greenery were purchased, and it did not include confierous trees. (Not that subdues some people who insist that there were, of course, Christmas trees from pagan times, off in corners where no records were made. . . sigh.)
And very few people know what the twelve days of Christmas are. This is not exactly a new observation, Chesterton, opening an essay with "These lines first appeared some time in Christmas week; thereby violating all the fundamental principles of modern civilization, defying the normal and necessary laws of Christmas Trade, Christinas Sales, Christmas Numbers, Christmas Shopping, and even a great deal of Christmas greeting; in a word, committing the crime of talking about Christmas quite near to Christmas Day." then went on to observe
Modern men have a vague feeling that when they have come to the feast, they have come to the finish. By modern commercial customs, the preparations for it have been so very long and the practice of it seems very short. This is, of course, in sharp contrast to the older traditional customs, in the days when it was a sacred festival for a simpler people. Then the preparation took the form of the more austere season of Advent and the fast of Christmas Eve. But when men passed on to the feast of Christmas, it went on for a long time after the feast of Christmas Day. It always went on for a continuous holiday of rejoicing for at least twelve days; and only ended in that wild culmination which Shakespeare described as Twelfth Night or What You Will. That is to say, it was a sort of Saturnalia which ended in anybody doing whatever he would; and in William Shakespeare writing some very beautiful and rather irrelevant poetry, round a perfectly impossible story about a brother and sister who looked exactly alike. In our more enlightened times, the perfectly impossible stories are printed in magazines a month or two before Christmas has begun at all; and in the hustle and hurry of this early publication, the beautiful poetry is somehow or other left out.
And you get other customs, like the Lord of Misrule and other forms of topsy-tervydom that were widespread, or the Victorian one of ghost stories. A Christmas Carol was one of course but so was Turn of the Screw, which had no references to Christmas at all, only to ghosts.
Even the folklore changes. The Tailor of Gloucester still remembers that animals can speak during the night, but that's not much used in modern tales. I sometimes hear people urging writers to go to non-European folklore on the grounds that European has been played out. It's barely been touched, if you skip the cliches that everyone uses.