I'm glad this thread has been started since I'm in the same boat too. (OK not first Christmas since diagnosis but first Christmas since taking things seriously). It's not the main Christmas dinner which I'm thinking about - I should be ok just having less (or maybe just one) roast potato. It's more the snacking that's a potential issue, the constant conveyor belt of chocolates, nuts, etc.
I don't really want to eat 'bad' things though, and undo any good work from losing weight and getting bg levels under control these last few months.