Summer runs from June to October, which is also the rainy season. It has the highest rainfall and highest temperatures in most places although the intensity depends on the region. The peak holiday months are July and August. Along the coastal areas, the hurricane season is from June to October (peak at August to mid-October). Winter is from December to April and is the traditional high season. Especially Cancun and the Riviera Maya become crowded; the resorts are packed at this time and prices go up drastically.
Taking all this into account, if you go to the coast, November is the ideal time to go. Rainy season and hurricanes should be over, water temperature is still pleasant (sea temperature of 26ºC – 29ºC). And it is not so crowded yet. Downside is that you’ll just miss out on Whale Shark Season (mid-May till mid-September) at Isla Holbox, but Bull Shark season (November – Feb/March) has just begun in Playa del Carmen!
So it all depends on what you are looking for. To summarize the best seasons per area and/or event:
- Mexico’s Pacific coast: August to February.
- Cancún: May to August
- Riviera Maya & Cozumel: November to March/April
- Whale Sharks at Isla Holbox: mid-May till mid-September
- Bull Sharks at Playa del Carmen: November till March