I found this on the list of possible Narnia books on another thread;
The Origin of Christmas: Other-Worldy Holidays
This is something I've been wondering. At first, one might think: how could Narnians now about Christmas, or call it that? But it might not be otherworldly at all. In fact, perhaps Christmas originated in Narnia and spread to our world. Narnia, after all, is ruled by its own version of Christ. Sure, he's called Aslan. But Christ is a title, not a surname. In Narnia, Aslan holds the title of Christ, just as a Yos'wa (Jesus) does in this one. Yes, Jesus Christ is also implied to be really the same being in both (and probably all) worlds, as implied at the end of Dawn Treader( and (becuase there can be only one Jesus), but I won't go into it that far. Christmas in Narnia, then, must celebrate, if not the birth of Aslan, then hid return, or perhaps his creation of the world. In any case, Christmas is a celebration and honor of Aslan in Narnia. It only makes sense. or perhaps Narnians are aware of Aslan's human form in this world, and that's how they got the name.