The weather, the crowds, the seaweed, and the price all move on the calendar. Here is how the year actually breaks down. Temperatures are approximate and shift a few degrees by source.
Dry Season
November to April
The marquee window. Highs in the mid-80s Fahrenheit, lower humidity, little rain, and the clearest water. This is also the most crowded and most expensive stretch, peaking from mid-December through spring break. February is the single best-weather month most years.
Rainy Season
May to October
Hotter and more humid, with short afternoon showers rather than all-day rain. September and October are the wettest. This is also when sargassum and hurricane odds are highest. The upside is real: the lowest crowds and the best resort deals of the year.
The Sweet Spot
May and November
The shoulder months are where I send value-minded travelers: good weather, thinner crowds, lower prices, and lower hurricane risk than the August-to-October core. You give up a little certainty on rain for a noticeably better rate.
On sargassum specifically: the seaweed season runs roughly April through October, heaviest May through August, and November through March is the low season, with the calmest, clearest water from December to February. Playa del Carmen's coast faces the open Caribbean with no offshore island to blunt it, so in a heavy week the central town beaches take the brunt while the beach clubs rake every morning. It varies week to week, so if you have already booked, send me your dates and I can usually give you a current read on your stretch of sand. The live beach webcams are the fastest way to see it yourself.
One seasonal highlight worth timing a trip around: from roughly June through September, whale sharks gather off Isla Mujeres and Holbox, north of Cancún, and licensed tours take small groups out to snorkel alongside them. From Playa del Carmen it is an early-alarm day trip, and worth it. It only happens in summer, which is also the quieter, better-value side of the year.