If what you want is to party, March is the best month for you since the so-called spring breakers visit us, in the month of April it is the so-called Holy Week in Mexico, probably the beaches are more crowded than the normal thing.
If you want to take care of your economy, the cheapest time to travel would be September and October, since it is the well-known low season, and it is when hotels and parks put more accessible prices although it is the time of hurricanes or some cloudy days.
If what you are looking for is the cheapest season, these months are the ones for you. But if what you are looking for is the best climate, we consider the perfect climate from November onwards because we do not have such intense heat and the afternoons and nights are fresher, although these dates have considerably high prices.
If you want to know how the weather will be, you can go to this section: