There are a few dishes that you will find at most homes for Christmas dinner in France. The first of these is foie gras. Foie gras is very expensive, and thus, is only enjoyed on special occasions. Though the process for how foie gras is "made" is gruesome, the final result is something that many people enjoy a great deal. It is cooked duck or goose liver, and the final product is very creamy and very fatty--almost like butter. The foie gras is traditionally served with Sauternes, a dessert white wine from the Bordeaux region of France.
So how is foie gras made? The ducks or geese are force fed, and they aren't permitted to move so that they can get really, really fat. Their liver is then cooked and put in a mold, giving the foie gras a pleasant shape that can easily be sliced. It is normally served with a small piece of toasted bread.
There is a lot of controversy about foie gras because of the inhumane practices used to fatten the ducks and geese. However, force feeding birds is not a novel practice. There are records that ancient Egypitians (as far as back as 2500 B.C.) force fed birds.
Another classic dish that French enjoy at Christmas is des huitres--oysters. The oysters are eaten raw with only a squirt of lemon juice. Raw oysters are an acquired taste, so most people either love them or don't.
Many people also eat saumon (salmon).
Lastly, you will find a buche de Noel (Yule Log) at pretty much every dinner table.