Sri Lanka has a warm tropical climate, with the coastal areas influenced by two monsoons - one along the southeast coast and one along the northeast coast. So there is always one side of the island that has good beaches. The sights of the Cutural Triangle, which are inland, can be visited at any time of year.
From December to March, the best beaches are along the southwest coast. This is also the whale watching season, which runs from November to April.
From April to September, the best beaches are at Trincomalee and Pasikuda in the northeast, which have the best snorkelling & diving.