Study the chart below which lists the caloric value of food and drink which will show up at the festivities. If you've done much cooking, you know where the higher caloric content in the festive foods comes from. Fat is what makes the food calorie-dense. Most of this fat comes from butter, cream, shortening, lard, beef fat such as the suet in mincemeat and plum pudding, and the oil in nuts and olives. Knowledge is power.
Planning helps too.
You can still nibble if you choose those nibbles with the chart in mind. Don't deprive yourself completely or insult your favourite aunt by refusing every morsel of the splendid seasonal baking that she is so proud of. You can control your intake, taste small portions of the special holiday foods and still enjoy the social season. If you do fall off the wagon, don't use that as an excuse to give up completely and binge your way through the month of December. In the cold light of the next dawn, read over the calorie and exercise charts and get back to your seasonal strategy.