Christmas Eve is magical just because it comes right before Christmas and all the stuff everyone has been looking forward to, thinking about and planning most of this month (if not before). Most families have Christmas traditions and rituals of one kind or another. Think of your own.
Now, think about holiday and seasonal rituals and how some one from another culture, another planet even, might see all the things we do for and around Christmas. What would seem to be the strangest thing of all? Write about it from the point of view of an alien or time traveler who has never heard of Christmas.