Are cheat meals allowed when on a diet ? like lets say once a week but only for one meal, not a whole day?

Not unreasonable: Obviously the more strictly one follows a diet the better the results, but you should still have very good results if you cheat only once a week for one meal- and if it makes it easier to stay on the diet it will be a valuable thing. The trick is to maintain a limit on cheating as it can be a slippery slope. And even when cheating try to be moderate.