What's a better time to start losing weight than new year ? You make resolutions, you make a plan and stick on it. Why wait for new year when you can start right now ? Yes, you can start losing weight right now, during Christmas.
Starting a diet or weight loss plan in January is easy because the month is not busy with events, it seems like a fresh start of the year. This situation fades out very quick: you will get busy and other events are coming up. Your plan is not taking the new situation into account because you did it when it was very easy to implement it, not taking into account things that might come up.
If you start your diet and apply your plan during Christmas holidays, you prepare yourself to be disciplined during rich meals and busy periods. If you can get trough Christmas meal and New Year party, the busiest times during the year, you will be prepared to face any challenges that will come during your plan.
If things go from easy to hard ( when you start your diet in January ), it is very unlikely that you can keep it for a long period. On the other side, if you start when it is hard and then come to easy periods, you can do it for a longer period.