MNT has the details:
“Around the world, weight gained from holiday feasting takes months to lose, a study found.
Christmas Day in particular is a holiday that appears to pack on the pounds: in a study of some 3,000 individuals in three countries, Americans showed an average 0.4% weight gain from 10 days before Christmas to 10 days after; Germans gained 0.6% more weight; and the Japanese 0.5%.
U.S. participants packed on 0.7% more weight in total during the full Christmas-New Year holiday season, but the Germans had us beat with a 1.0% weight gain, according to Brian Wansink, PhD, of Cornell University in Ithaca, N.Y. and colleagues.”
Source: Holiday Feasts Take Months-Long Weight Toll | Medpage Today
Those percentages aren’t very helpful, are they? In real life, if you weigh 180 lb (81.8 kg) and gain an extra 0.7%, you’re all the way up to a whopping 181.26 lb (82.4 kg). But if you do that—1.26 lb—every year for 20 years and fail to lose the weight, you’re up to 205 lb (93.2 kg) and now you’ve got diabetes and high blood pressure.
Here are a few tips to avoid the weight gain:
- On the day of the major feast, just eat two meals, and make one of them small
- Don’t snack or graze; just eat at mealtimes
- Work in some extra exercise
- Minimize the alcohol that weakens your discipline
Steve Parker, M.D.
Pro Tip: Read one of my books before you make your annual New Year’s weight-loss resolutions.
PPS: Click for the research report in NEJM.
Paleo Diabetic 
During Thanksgiving, Christmas Eve and Christmas Day I eat everything in sight. However, the day after I go into a fast and the next day I go back to my LCHF eating style. Usually all weight gain is gone within a week. The bottom line is that nobody becomes fat in one day.
I’m planning to LOSE weight over the holidays this year. I’ve been seriously LCHFing for the past 2 or 3 months with a bit of IF thrown in, and am down about 12 lbs, yay! Today I have ramped it up with a plan for a 36-42 hour fast 2 or 3 times a week, and tightening up the LCHF. On Thanksgiving I will consume higher carbs (eating at my sister’s house), but the days before and after will be fast days. Christmas will be tougher, since I’m obligated to host Christmas Eve, but have no control over the menu (everyone is bringing an Italian dish!), then back to Sis’s for Christmas. Thanks to IF, however, I expect to sail through the holidays and come out the other side a few lbs lighter!