1 cheat day is ok when trying to lose the fat, I wouldn't load any more than 4000 to 4500 calories that day though, assuming you're under 205 in weight class.
I don't know above that point.
Think about it like this though. What if you were eating 2000 calories a day (normal for a medium sized person.) and one day a week you only consumed 500 calories. Would that make a difference in weight?
But absolutely going balls to the walls on the day off is bad because it might affect your weight, which would be minimal, but still annoying, but the big reason would be the effect of feeling full of **** and sluggish if you were training the next day.
Oh, and if you're one of those guys who fluctuates 20-30 or so pounds of fat between competitions I would advise not to. I am a fridgenator too but mass fluctuations kill your longevity.
Just adding that... now I'm off to the fridge, J/K lol