i cheated on thanksgiving and it didn't affect me too bad the first day or so, so i kept on going thinking i could eat whatever i wanted and it hit me within a week.
you just need to suck it up and get back on the diet! once i started feeling really crappy i got SUPER strict again. it will take a few weeks to get back to feeling more balanced though.
i would suggest starting january 1st since the holidays are tough and you already fell of the wagon. that is just my personal opinion though. if you are terribly ill then you should start ASAP!