I actually have a bit of a different concept here. I HATE the idea of a "cheat" ... whether it's a day, a meal, or whatever.
A cheat is a shameful thing, something to be hidden; it's deceitful. The dictionary defines a cheat as a "fraud" and as "unfaithfulness". So if you "cheat" ... what are you cheating on? Yourself? Your life? Why set yourself up for the guilt and shame that follows when you "cheat" yourself?
I don't cheat. I don't always eat 100% healthily; very few people can achieve that. Our lives are full of things that aren't always healthy - there's always a birthday or a evening out with the guys/girls, or an office function, or a potluck, or something. When you go to those things and *make the choice* to eat something unhealthy, then you should do it taking full responsibility for it and accept that you are treating yourself.
TREAT, not CHEAT.
I choose when I eat something unhealthy - when I have a few pieces of pizza, when I drink a margarita, when I have cake or cookies. Those aren't cheats - because the only person I could cheat on is myself. Those are treats that I choose to have and choose to account for.
Don't cheat yourself.
It's not worth it.