"Practise saying no
Make a plan for each event."
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It's just a Christmas (not holiday) party/dinner. Why prepare for it like it's a job interview? If you are overeating or eating unhealthy most of the time abstaining for holidays won't make a difference. If you don't, chances are you don't need to do anything special. 

And don't be the odd one at the table/party pointing to each dish and asking "what's in that? oh, I can't eat carbs". Eat or don't, jut don't involve everyone in your personal challenges.