You won't have "gained" weight from a day or two of festive eating. If you do over indulge, especially on a lot of carbs, sugar, sodium etc you retain more water (as well as the undigested and digested food in your system), and the excess causes an increase in the scale which quickly resolves itself. In order to gain one kg even you need to eat an EXCESS of 7000 calories (3500 calories is one pound). So you didn't gain over Christmas it was just a water weight fluctuation.