Depends:
How much "overeating" are we talking about (twice your daily calorie needs? a hundred calories extra?)?
Does this happen on a regular basis?
Is it REALLY only one day a week (be honest with yourself)?
Some say that overeating every once in a while is a metabolism trigger for the body, that helps up the metabolism.
Some people are using a variable-calorie diet system (goes by another name, which I forget) in which they cycle the amount of calories they eat: somedays "undereating," some days "overeating." It all evens out in the end, and it keeps the body from going into starvation mode and hanging on to fat after the low calorie days.
I've also read that the body can't process more than about 500 calories at a time (what it can use for it's immediate needs), so any calories over that per meal are more apt to be stored rathen than burned (also depends on the person: the number would go up for a big, muscular guy, who would burn more, and down for a tiny woman, who would burn less).
If you find yourself consistently overeating one day a week, try to focus on eating more (if you want), but of nutrient-dense, lower calorie foods, like big salads with lots of greens (you could eat a huge amount of greens without eating a huge amount of calories).
