Mérida has a hot, humid tropical climate. The year divides into a dry season (November to April) and a wet season (May to October), with the heat varying less than the rainfall. The city sits at low altitude on the flat limestone plain of the Yucatán, which has no rivers and no cooling breezes from mountains; heat and humidity are the baseline conditions.
The best months to visit are November through February. Temperatures are still warm by European standards (daytime highs around 28–30°C), but the humidity drops and nights are genuinely comfortable. This is also peak season, so the city is busy, hotels charge more, and the best restaurant tables fill up. March and April are also viable; temperatures begin climbing (reaching 38–40°C in April) but it is still dry.
May through October is hot and wet. The afternoon rains are heavy but usually brief — a downpour from 4–6pm, then clearing — and the mornings can still be pleasant. Hurricanes occasionally affect the Yucatán from June to November, though Mérida itself, inland, is less exposed than the coast. The wet season is low season: prices drop significantly and the city is less crowded. If you are heat-tolerant and budget-conscious, it is a perfectly reasonable time to go.
The main cultural events worth timing a visit around are the Vaquería (traditional Yucatecan dance festivals, held at various points through the year but particularly in spring), the Otoño Cultural season in October, and Hanal Pixán (Yucatán's Day of the Dead celebration, in early November) — one of the most elaborate in Mexico, with elaborate ofrendas set up in public spaces and the cemeteries illuminated all night.