The peak season runs from November to February: dry, sunny days, calm seas, all the beach shacks open and staffed, and the main beach busy with domestic and international visitors. Prices for accommodation and transport are at their highest. December and January are the most crowded months.
October is a good compromise. The monsoon has retreated, the crowds haven't arrived, and — if you time it right — you can catch the Fama of Colva festival, when the Church of Our Lady of Mercy fills with pilgrims and the village feels briefly like a different place.
March and April see the season winding down: cheaper, quieter, and still reliably warm, though the heat begins to build toward the end of April. May is very hot.
The monsoon (June to September) is not a beach season. The surf is rough and swimming is unsafe, most beach shacks close entirely, and heavy rain is frequent. Some visitors come for the green lushness and the low prices, but they're not coming for the beach.