The best time to visit Mirissa is from November to April, during the northeast monsoon season when the south and west coasts are sheltered and the sea is calm. December through February is peak season — the weather is reliable, the whale watching is excellent, and the beach is at its most inviting. This is also when prices are highest and accommodation fills up quickly; book ahead.
The southwest monsoon arrives in May and brings rough seas to the south coast from May through October. The beach becomes unsafe for swimming, and whale watching trips stop running. Mirissa empties out considerably during these months, and prices drop to match. If you are travelling during the monsoon, the east coast (Arugam Bay, Trincomalee) is in its dry season instead.