San Carlos has two distinct seasons. October to April is the comfortable period: days are warm and sunny (20–28°C), nights are cool, the sea is calm, and the water is clear. This is when the expat community swells, grey and blue whales pass through, and the diving is at its most predictable. American and Canadian snowbirds account for most visitors during these months.
May to September is hot. Temperatures regularly exceed 35°C and can push 40°C in July and August. The Sea of Cortez becomes a warm bath — water temperatures above 30°C — which is pleasant for swimming but reduces diving visibility somewhat. The big pelagic fish (marlin, dorado, sailfish) are most abundant in this period, so serious sport fishermen often prefer summer despite the heat. Humidity is low most of the time, but late-summer tropical storms occasionally brush the coast.
December and January are the coolest months and the most reliable for whale watching. The Christmas and New Year period sees a significant spike in Mexican visitors from Hermosillo and Guaymas. Book accommodation in advance for that window.
There are no major festivals tied specifically to San Carlos. The nearby port of Guaymas holds a well-attended Carnaval in February — one of the largest in northwest Mexico — which is worth timing a visit around.