I don't know if it's officially on the calendar but I think we are officially in the Dead of Winter.
And the Dead of Winter can be depressing.
Think about it: Christmas and New Year's are way over. A week ago, hearing an occasional Christmas carol or seeing some Christmas lights up wasn't the end of the world and in fact may have still made you happy. Now, it's time to change the CD or get the ladder out and take down those lights.
Spring is but a distant memory, even here in sunny Southern California. Sure, it's 70-80 degrees in the middle of the day but we've had terrible winds and it's still cold (for us Cali folks anyway) in the mornings and at night. March or April is way far away from this point.
So, it may be easy to revert back to some old habits. When the weather is terrible and when the outside world seems to have deserted you, it's easy to just say "Forget this" and go back to the way things were.
But you mustn't. Now might be the most difficult time of all, when the weather's against you and few else are for you. It might impede you from running outside for instance or going somewhere due bitter conditions. But that need only be an obstacle to your fitness/nutrition plan.
Even if the weather is cold, even if the calendar says no holidays until whenever, no Spring until whenever, it's still time to stick to your goals. Otherwise, you probably won't look or feel the way you'd like if and when the warm weather finally arrives.