Don't make excuses: "I don't have time during the holidays." "I am too tired." "I need to relax during the holidays." "I'll start again after New Years." "I'll make up for it."
Be true to yourself and the healthy lifestyle you are choosing! Sure you can have a piece of pumpkin pie after Thanksgiving dinner, but don't eat more than a slice. Make your favorite foods healthier with whole grains, substituting yogurt for mayo or sour cream, bring a healthy appetizer to share, and keep your exercise routine in check.
Last year I signed up for a half-marathon in January to stay motivated through the holidays and to keep on track during the cold MN winter. It wasn't always easy to get out in sub-zero temperatures to go running, but I never regretted it when I got home.
Don't make exceptions. Be the exception! You are worth it!