At least most of the time, the weight you gain over Christmas is water, this is important to remember for the sake of your mental state, and as you know your mindset is a massive factor when it comes to kick starting your New Year! You should not allow yourself the luxury of falling into a self pitying sate of mind, this is damaging to your being, instead take a moment to step back and look at the bigger picture! For example, have you really gained fat over the Christmas period…I mean, yes, the scales might be suggesting that you have, but remember that scales are indiscriminate i.e. they do not consider body composition. As a Dietitian who has advised many a person after a Christmas binge, I can tell you that the bulk of the weight gained in that week will have been water. This water retention can be mostly attributed to the increased carbohydrate consumption over the Christmas period. On average, for every 1g of carbs you consume you retain 3g of water, this can be subcutaneously (under the surface of the skin) or intramuscularly (in the muscle).
So DO NOT let yourself get down during this New Years period because of the 2,3,4,5kg you gained over Christmas. Give yourself some time and adjust the carb intake so that it resembles approx 3g carbs per kg bodyweight, and watch the weight reduce as if by magic!