November to March is the most comfortable time to visit — daytime temperatures in the high 20s°C, low humidity, cool evenings. April and May are very hot, often above 35°C with little rain; the cenotes become a necessity rather than a luxury. The rainy season runs roughly June to October; storms are short, frequent, and tropical in intensity, but the green of the surrounding jungle is at its best then.
Time your trip to land on a Sunday if you can: the town closes the streets around the plaza for a small market and an evening of live music, and the atmosphere is at its best. The Equinox at Chichen Itza (around 21 March and 22 September) brings huge crowds — Valladolid fills up, prices rise, and the cenotes are busy.