It's Saint Nicholas Day, December 6, earliest sunset of the year, so I think I'll talk about Santa, Jolly Old Saint Nick, and the question of how the saint whose feast day is almost three weeks before Christmas has gotten associated with Christmas. (Hate to break it to you, but medieval Christmas was completely lacking in both Santa and Christmas trees.)
(And for that matter, isn't "Old Nick" another name for Satan? And if you rearrange the letters in Santa, what do you get?)
There really was a Saint Nicholas, by the way, who lived in what is now Turkey in the later days of the Roman Empire. He had various stories told about him, including that he saved some girls whose destitute father was going to sell them to the local brothel; the saint saved them by secretly throwing bags of money through their window. He also supposedly brought back to life some boys whom a nefarious innkeeper had cooked up and was getting ready to serve. These stories made him the patron saint of children. (See more here on how he became the patron saint of poor working girls during the Renaissance.)
By the seventeenth or eighteenth century, well-to-do Dutch were celebrating Saint Nicholas day (Sint Niklaus or Sinter Klaas as they called him) with small presents for the children. In the Netherlands, the saint still comes up the canal in his canal boat on the 6th, bringing toys for the good children, but his assistant, Black Peter, puts coal in the wooden shoes of the bad ones.
Santa as we know him (say Sinter Klaas fast and you'll see where the name Santa Claus comes from) is a specifically American invention, that started in the early nineteenth century with Washington Irving, the same person who wrote about the headless horseman of Sleepy Hollow. He was trying to create a vision of an "old fashioned" sort of family celebration, based on Dutch heritage--with the war of 1812 and all that, the British weren't as welcome in New York State as the Dutch still were.
But it was Clement Moore's poem "A Visit from Saint Nicholas" (aka "The Night Before Christmas") that single-handedly moved Santa from December 6 to the 24th. By the middle of the nineteenth century, Santa was well established, driving out alternate gift-givers. Thomas Nash, a newspaper illustrator (whose Santa is above), was the first to come up with the North Pole toyshop--I guess someone dressed in furs and driving reindeer just had to live in northerly climes.
But the jolly, red-dressed figure, with a bushy beard, big black boots, and a sack of gifts only really took the appearance he has now in the twentieth century, with Coca Cola commercials, where he chugged a frosty bottle of Coke, rather than puffing on a Dutch clay pipe as he had earlier (and is doing in Nash's illustration).
The Byzantine saint certainly has had many transformations in the last 1600 years, to being considered a secular symbol of Christmas, acceptable where a manger scene would not be (wait! aren't saints religious by definition?), most visible sitting in the mall in December where terrified children are forced to sit on his lap. Ho ho ho!
For more on Santa and Christmas in the Middle Ages, see my essay, "Contested Christmas," available on Amazon and other ebook platforms.