The Yucatan Peninsula is hot year-round.  The two distinguishable seasons are the wet season and the dry season.  The wet season usually starts in June and ends in October, making the dry season generally from November through May. 

The weather here in the dry season is much more desirable and pleasant for most tourists.  The dry season is still hot, but much more tolerable than the wet season.  Although the average temperatures throughout the year don't vary by much . It usually gets about 74°F (23.5°C) in January and  82°F (28°C) in July. The humidity in the summer months makes it feel excruciatingly hot most days, especially to those who aren't used to a humid climate.  In the wet summer, it actually rains most days.   This rain comes in heavy, short spurts usually towards late afternoon.  There is also a strong threat of hurricanes during the wet season (which is why the wet season is often referred to as hurricane season), which have been getting stronger over the past few years.  If the low prices of the summer attract you enough, make sure you completely understand your risks weather-wise before you plan your trip.