Decorate your Christmas tree! This can mean something different to each family who celebrates Christmas: you might hop in the car and head out to a lot, bundle up and cut your own on a tree farm or go up to the attic and bring down your tree-in-a-box. Some people decorate an interesting "found" branch they've stuck in a bucket of sand, and others have a roll-up felt wall-hanging tree with felt ornaments.
Whatever "The Tree" means to you, its set-up, location and decoration are probably governed by long-standing family tradition. For example, we use a lot of lights on our tree - and I mean a lot, one year we had more than 25,000 twinkle lights on it and oh, what a tree that was! - so light placement usually takes place over the course of a day or two. When they're all on the silver-and-gold bead garlands are draped. Then the ornaments and candy canes - and, this year, tinsel!
Play Christmas music while you work. Serve eggnog when you're finished. Take lots of pictures. Don't worry if the red balls are all on one branch or the left side of the tree is bare - who cares? It will look beautiful anyway (and if it really bothers you, you can "fix" it after the kids are in bed).
About ornaments: we each make a new one every year (this year we stitched our own designs onto plastic canvas squares using yarn - the kids will take their homemade ornament "trousseaus" with them when they move out) and I buy a new one, too (this year I added to my Star Santa collection, but I'm hoping to find a red glass bird as well). There's also a glass pickle ornament that I hide in the tree while we're decorating; the first one to find the pickle wins!
I confess that since a) we have a houseful of kids who can't wait and b) we use a fake one, we put our Christmas tree up a day or two after Thanksgiving. Of course we don't decorate it until everyone can be here - something that gets more and more difficult to schedule each year, as people grow up and get jobs and lives - but it's one of my favorite parts of the whole season. If there is a gaudier piece of oversize jewelry than a Christmas tree, I don't know what it is; and its twinkling, shining presence in the house is a visible link to Christmases past, Christmas present, and Christmases yet to come.