The first thing you could do is offer to host this way you can choose the foods offered. Here are a few cute and festive ideas for party platters.
Fruit Christmas Tree from Jeanne Benedict (pic above)
Creative Kids Snacks, Grinch Party Poppers
Also from Creative Kids Snacks, Santa Party Poppers
Strawberry Santas from Kids Activities Blog
Strawberry Santas Hats from Snack Girl
Christmas Tree Fruit Platter from The First Year Blog
If this is too much effort, then make and take one of the above with you. This way you have healthy options, and everybody can praise your creativity. Win-Win!
Ok so you don't have time, or aren't creative enough to pull off those platters, then do your best to make healthy choices when eating.
Try eating before you go to a party or drinking water to fill you up so you won't feel compelled to snack.
Talk to your family and friends about your goals ask if they could put out some healthy options along with that fudge and peppermint bark. If they're supportive they should understand.
Plan to fail. I know how this sounds, but sometimes you have to just give in. Try to eat just one or two treats, make a plate and share it with a friend or your partner.
Most importantly don't beat yourself up if you give in and eat more than you thought you would.