Why do you gain weight after a holiday meal? Well, the obvious reason is that holiday meals are loaded with calories, but since it takes roughly 3000 excess calories to create a pound of fat... The calories aren't responsible for the entire 3-4 pounds you put on the day after Christmas. Actually, water and your kidneys are primarily responsible for that.
You see, holiday meals often consist of a lot of salt. Your kidneys are responsible for regulating your body's salt concentration. Kidneys regulate salt in two ways, one, they decide to excrete more or less salt out of your body through the urine, two, they decide to excrete more or less water out of your body through the urine.
Kidneys regulate the body's water on a minute to minute basis, but they regulate the body's salt on a day to day basis. So, when you take in a ton of salt during that thanksgiving meal, your body's salt concentration immedialtely increases. Your kidneys' respond initially by retaining more fluid than they normally would (meaning you pee less). This extra water returns your body's salt concentration to normal, even though your body's absolute salt levels have increased (I.e. You are retaining water). Then over the next few days your body pees out the excess salt and retained water, keeping your salt concentration normal and bringing the absolute amount of salt in your body back down to normal levels. When you lose the water, you lose the weight.