Personally I would have christmas day off the diet.
I see it about weighing up the impacts, eating christmas dinner would mean having to extend my time on CD by, hmmm, maybe a week. Well, up to a week. I don't find CD that bad in general, so I would be prepared to extend it if it meant having a completely lovely christmas with a great christmas dinner.
Having said that, I would probably avoid pudding!!
Dee x