The feeling after New Years for the first part of January where you realize the holidays are officially over and not only will you have to wait 11 months to re-experience the joy you felt during this time, you have to return to the norm (school, work, etc.) knowing there won't be much to look forward to for a while unfortunately. Usually lasts about a week to a month, though extreme cases may not wear off until spring.
Similar to Holiday Withdrawal
Poor Tom. He's started his holiday comedown knowing Christmas is over and he has to go back to school on Monday.