The holiday gathering at work is always an odd event. You work with your colleagues all day, then go and eat and drink with them all night. When you get to work the next day (because they're always on a work day), you have to see them early in the morning and anything you regrettably said or heard is still hanging there. Plus everyone's hungover, but they all have to act like they're not hungover. That one day-after holiday party accounts for more lost hours of productivity than all of cold and flu season.