Obviously it will be weird for me to be eating different food than my family, but since I'm vegetarian, it happens anyways so I use that as my excuse to make my own food. It helps that my immediate family knows and are supportive/know that I'm 18 and there isn't much they can do.
How to get through a three course Christmas meal:
I make myself a very nice garden salad and fill up on this. I don't dress my salads, I simply put some salt and pepper on it. I often have the same salad my family is having, and ask them to leave the dressing on the side. I'll tend to skip the bread, when the bread basket goes around, or put a very small piece of bread in the basket and go for that one. Bread is such a fear food for me, I'd rather not be triggered to b/p right from the get go.
Salad ~ 50 (about two-three cups worth, no dressing)
Bread ~50 (for a small piece)
2. Main course.
I replace the turkey with tofurky roast (125 per tenth of the roast) and then have a plateful of steamed veggies (about 50 for two cups). I can't do potatoes, so I always skip those.
This is the though one and seems to ALWAYS lead me to binging and purging. I plan on loading up on fruit salad (about 80 for a cup) and some light whipped cream to look like I'm indulging (25 per quarter cup)
In total your meal can easily be under 400cals, and you still feel part of the Christmas celebrations.