Before I'll start writing my own Christmas tale I will tell you how to write one. Of course there are many ways.
Classic
If you don't have a original idea this might be the right option for you. Before you start writing read classic tales. Do not copy them. Try to catch their spirit that people love year and year out. I think classical story could and should still happen in modern era. Like I said before spirit is hard to capture because it's the heart of the story. That's it, classic tale needs hearth.
Ironic
This one should be original. If you have found some ironic aspect of Christmas you should hit it. Be careful that your story doesn't turn in to preaching. That is always the trouble when you feel strong about the subject. Typical subjects are forgetting of Jesus and rich people giving rich people material things. If you can find some other subject, don't start preaching and create an clever story with interesting character you have a ironic masterpiece in the making. Commercially this is not going to be a hit. Rarely would anyone buy this as a gift.
Modern
This is a story set on Christmas and Christmas is a great theme. Story should have something new, something that makes it different. This is likely hardest to write because you can't rely on previous work but you can't completely leave them.
Some kind of an adventure involving Santa Claus is typical but of course you can make it interesting with a little imagination.
Hopefully found this piece helpful or at least interesting.
One more tip: Go out during shopping season. You might get ideas.
