Why not just choose a workable diet, that is something you can do every day, and forget about cheat days?
The answer is if you want to make a lifestyle shift then cheat days are part of a workable diet. Most of the really good nutrition sites I use say that about 10% of your meals can be cheat meals, and the reason is that we don't just eat food as fuel, that is simply a myth, food also is satisfying, and if you don't include 'fun foods' in it, you won't maintain the lifestyle shift. Diets don't work because of that, they are too stringent, it is why severe calorie counting and the like ending up failing.
So for example, if you choose to eat a diet where you restrict grains, then having a hamburger or a pastrami sandwich once a week isn't going to kill you. Likewise, if you eat a good meal (vegetables, salad, some lean protein) but then share a slice of cheesecake with someone, it won't make you fat or kill you. Julia Child hit the nail on the head with that, she said if you eat rich foods, eat a small portion and enjoy it, rather than suffering through some macrobiotic bleh that an Alice Waters turns out, and I agree. Long term, by 'cheating', you can sustain eating. A diet is something you do to lose 20 pounds; a lifestyle change in eating is for the long term
One side note, doing crunches or situps won't get rid of belly fat, it will build muscles under the fat which may make it look worse, it will bulge out. Situps and crunches once you have your body fat percentage down can work, but in the meantime, ditch them.