Crab on Christmas either means you've gone to great expense to get king crab that was flown to you or you live on a part of the West Coast where Dungeness crab (pictured above) is in season and cracked crab is a holiday tradition.
King crab legs are sold cooked and often cracked. Serve them with plenty of melted butter and/or aioli for dipping. You can steam them to gently heat them up, but know that you risk over-cooking them and turning the meat rubbery or tough.
Dungeness crab are sold live or cleaned and cracked and ready to eat. Buying the ready to go is, of course, the easiest option, but cooking and cleaning live crabs is easy and is less gross than you may think. See How to Clean Crabs.