So, I know that people have expressed that they think that this book is a Christmas story because Father Christmas shows up, and while that is a super big clue towards us looking at it as a Christmas story, it's much more than just a fun cameo.
You see, Jadis made it always winter and never Christmas. As someone who lives in MN, which means that it's winter usually at least 6 months out of the year. The only good thing about winter is that Christmas is coming!!!!
Also keep in mind, that winter, though it can be fun, often isn't. Since my house is heated with wood, I've had to get up early in the morning, or stay up late at night to make sure that my family stays warm. (I'm not the Dad, btw, it's more just a chore) There have been times when our family snowplow has broken down, and we've had to shovel out snow drifts that are five feet tall. This isn't a pity party, it's just case and poit, winter can be tough. And like I said, Christmas is the best part of winter.
So, if it were winter, and never Christmas, then that would mean that the best part of this mostly uncomfortable thing would be gone. In other words, the hope of winter would be gone. The hope of CHRISTmas would be gone.
The White Witch's rule has caused the hope to be gone from Narnia. There is no positives to her reign. The other thing to point out is that many animals go into hibernation during the winter, and it's almost as if the spirit of Narnia has been put to sleep.
However, with the return of Aslan, and the fulfillment of the prophecy, it doesn't just elminate winter, oh no. It's much better than that. Instead, the Narnians, (and specifically the children) receive the benifit of the best thing that winter has to offer. They get to have a Narnian Christmas. People have pointed out that isn't it weird that the Narnians have Christmas, and my opinion is, "not really". First of all, we know that Aslan is known by another name in our world. (Jesu-) The other thing though is that there are certain things that come with the idea of "Christmas" that another name, such as Winter Solstace, or whatever, wouldn't carry.
The book really is about the coming of Christmas. Not just that one scene, but the entire book! Why so? Because the whole thing is about the return of hope, and joy, and love to Narnia, and that is what Christmas, and Christ, are all about.