I'm probably alone on this, but I adore these movies. They're probably the only Christmas-themed pop culture I will watch out of season. In no way does this mean I think they're good movies on an empirical scale, but that I really dig them. I think it has something to do with its idea of drawing back a curtain on a myth to prove that the myth could totally be true (with some magic of course).
The first one is where most of the magic and all of the set up of this "behind the curtain" aspect gets set up. Scott Calvin (Tim Allen in his heyday) is a crappy divorced dad who has his young son, Charlie (Eric Lloyd), for Christmas Eve. Charlie still believes in Santa, and gets confused by small things in "The Night before Christmas" (he thinks it's the Arose suchak ladder, instead of Arose such a clatter). That night Charlie wakes up when Santa and the reindeer land on the roof. Scott goes out to see what happened and scares Santa who falls off the roof, and disappears leaving his suit on the ground. There is a business card in the pocket that reads, "Put on the suit, the reindeer will know what to do". So as not to scar Charlie by killing Santa, he does and ends up delivering presents - in his boxers and Santa's jacket. The magic just sort of rolls off him (the empty sack refilling itself at a new house, the fireplace appearing so he can come out of it) and he and Charlie arrive at the North Pole and meet Bernard (David Krumholtz) the head Elf. Bernard's the one who points out that Scott Calvin has fallen victim to "The Santa Clause" - by putting on the suit you give up your right to any previous identity and become Santa Claus. Calvin is pretty skeptical, and when he wakes up the next morning, he's sure it was all a dream. However, Charlie believes - and because he tells his mom (Wendy Crewson) and step-dad (Judge Reinhold), they eventually take away Scott's parental rights. That's almost the least of Scott's problems as his hair turns white almost over night, his beard grows really fast and he puts on 40 pounds in a week (the Santafication process isn't pretty). Scott Calvin fights the whole process (dying his hair, etc.) but can't stop it when "The List" arrives and he actually knows who's been naughty and who's been nice.
The movie is filled with quick explanations for how the myth of Santa could actually happen, and why adults don't believe. I've probably seen this a few dozen times and usually child actors are what make it impossible to rewatch humorous adults. While there are some odd elves that I find annoying, mostly Charlie runs the show, and he's perfectly whiny, but innocent and full of faith, both in his father and in the magic of Christmas.
The second movie, subtitled, The Mrs. Clause sees Scott Calvin having to find a wife of eliminate the magic of Christmas. He returns to the real world with 28 days to go - but so as not to worry the elves, he leaves a toy version of himself in charge. Back in Charlie's house, mom and step-dad have created a half-sister, Lucy (Liliana Mumy). Scott has gotten pretty good at being Santa. He's taken his place on the council of legendary figures (Mother Earth, Father Time, the Easter Bunny, the Sandman, Cupid, and Jack Frost), and is pretty magical himself now. He falls for Charlie's Christmas-hating principal, Carol Newman (Elizabeth Mitchell), using a little magic to help her remember the magic of Christmas. Some shenanigans ensue when the toy Santa decides that all children are bad and makes the elves give everyone coal. Luckily Santa, and the future Mrs. Claus make it home in time to save Christmas.
The 3rd installment goes pretty far off the rails by attempting to do a "It's Wonderful Life" montage by making Scott Calvin realize what might have happened if he hadn't become Santa, but instead Jack Frost (a way too over the top Martin Short) has commercialized it instead. Of course things are much worse and Scott has to use his family and friends to save the day again. And this time Mrs. Claus is giving birth, and we meet her parents too - Ann Margaret and Alan Arkin, neither of whom know that Scott is Santa or that they live at the north pole. The subterfuge they use to fool them is pretty funny, but seeing Frost's version of Christmas is too much. Still a fun use of the franchise though. One of the best things about the entire trilogy is that they don't replace any of the actors (David Krumholtz doesn't appear in the final movie, but he's the only major player who doesn't) and do add some when necessary to build the story.