For most Americans, Thanksgiving is the healthiest meal of the year. It may be too much food, but it's one day when we eat homemade, real food. Enjoy it, eat more than you should if you like, then tomorrow is another day.
Are you really that hungry at midnight snack time? Considering how much we ate at Thanksgiving lunch and/or dinner, it's unlikely to much more than emotional eating, wishful thinking, or the excuse that "we already blew it, earlier." Every meal stands alone.
Even leftovers can be eaten in moderation. Aren't you surrounded by desserts, high calorie foods, and crap on all normal days, too? Just because they are leftovers you're going to overeat them now?
Every meal stands alone, but every day stands alone, too. Just because you eat too much today doesn't mean you can't get right back on track tomorrow. Do it. If you don't, realize that you're making an excuse for yourself. You better look into that...
Some of us have foods that make us feel like crap, whether it's dairy, wheat, or whatever. Don't eat it just because Mom made it. Figure out how to let her down easy. ...and next holiday, tell her ahead of time that you can't eat _____, so she won't be surprised. Enjoy the foods you can eat, you'll still likely be stuffed.
On the other hand, just because you don't allow yourself to eat white potatoes, added sugar, dark meat, or carbs and fat in the same meal, doesn't mean you can't eat them today. Our ancestors were fit and trim, for the most part, and they ate these things regularly. I believe that 80% or so of our health and dieting woes are primarily caused by chronic overeating, and the individual ingredient comes second.
These things are important, but not important enough to ruin one beautiful meal with family and friends. Please have a wonderful Thanksgiving, both the dinner and the day!