A cheat day is useful for a few reasons; the biggest of which is psychological. There are very few people out there who can live their lives without ever deviating from their clean-eating spectrum. I read something once where Eric Cressey said he had gone several years without eating a piece of cake, even on his birthday. That just makes me uncomfortable. I really like cake. Donuts make my knees wobble and pizza makes me drool.
My eating schedule is as such: Monday-Friday I eat as cleanly as possible. I stick to my carb-backloading schedule as much as I am able to and don't eat junk. Note: I may occasionally have a donut on the way to my workout on Friday afternoon. Saturday morning I work at the gym, and when I get done around 1 o'clock I let my hair down. I end up with a day and a half of eating like a civilian and whatever that entails. I don't guard myself against anything. A buddy of mine sticks to pretty much the same schedule and his Sunday morning meal includes a Starbucks bacon egg and cheese on a chocolate chip bagel. Yup.
I'm not saying you have to go that far, but you get the idea. If you're training consistently/intensely and eating cleanly then your body is going to assimilate everything you put into it.
The mental break that a cheat day provides is a lifesaver; after a long week of work and training, it's nice to be able to sit back and order a pizza and wings when you watch the game. Going to a barbecue with friends and having beer and a dessert makes going back to the Grind very bearable.
For those of you who maintain a caloric deficit during the week, a cheat day gives your body the opportunity to gain back some much-needed calories for growth and allows you a little bit of a rebound effect. Essentially, this flood of calories will stimulate your body to grow and recover.
Don't worry; you won't get fat. Your body fat percentage isn't going to go from 9 to 16 over the course of the weekend. One day of eating anything isn't going to make you fat. A month of it will, though.
Before you go out this weekend and crush a few pizzas, be honest with yourself. How good is your diet during the week? How hard have you been training? Cheat days are a reward, not a right. If your diet has been shit and you haven't lifted in 10 days then you don't really need to reward yourself for anything.
Think about it; cheat days are a good idea. Have a great day and go lift something heavy!
**For additional reading, check out John Romaniello.