Around 8, dinner can start. If it's for lunch, then it starts around 1.
Here is the typical dishes we eat for Christmas. If you invite some people, do not make them all.
Choose one or two entrées plus the salad, one main course + vegetables, some cheese, at least two desserts.
If you invite some people but you don't feel comfortable with all this cooking, just go to a « traiteur ». A traiteur cooks good quality food. It's a little bit expensive but it's worth it.
If you invite or are invited, after each dish, ask (or you'll be asked) if somebody wants some more : « vous en voulez encore ? », « je vous ressers ? ».
So, if you're the host, always make one or two portions more. But if you're the guest, even if it's good, don't forget you must eat of each course. It would be very unpolite to stop eating, even if you're very full.
How to refuse politely : « Non merci, mais c'est très bon ».
How to ask for just a small portion : « un tout petit peu s'il vous plait »