[shilala]

I don't think a cheat day is a good idea at all, if it's just for the sake of a cheat day. If something special comes along like an Anniversary Dinner of Graduation or whatever and the menu is out of the line of what one would ordinarily eat, then by all means, have at her. If the cheat day is simply for the sake of overeating, I think it's a really bad idea. It simply exists to help bad habits to remain.
They say it takes a month to change a habit. If I take a cheat day every week, that'll ensure I never get the habit changed, so it just dooms me to never getting used to a new way of doing things.
I'm sure there's other lines of thought, and everyone has their right to do what they see fit, but I can't see any productive reason for it, except for maybe getting the hook in someone, then doing away with the cheat day later. I don't even think that's a good idea.