During the low season the resorts have very attractive special offers with up to 40% accommodation discounts, transfer upgrades, meal plan upgrades etc.
If you are living in a hot country where summers are just unbearably hot (UAE, Qatar etc) you have a great opportunity to escape that heat and benefit from the special offers at the same time. If you live in a colder climate, travelling during the low season would still give you a proper summer, also at a great rate.
Travelling during Jan, Feb and March would give you more sun during the day but the rates will be higher too.
The most expensive period is from mid-December until mid Jan, this is peak season when the resorts have a festive program for Christmas and New Year, the rates are twice-trice or even more higher than they usually are.