Kalutara's weather follows the pattern of Sri Lanka's southwest coast. The best time to visit is from November to April, when the northeast monsoon brings dry conditions to the west coast. This is peak beach season, and the sea is generally calm and swimmable.
The southwest monsoon runs from May to October, bringing heavy rain and rough seas to Kalutara. The beach becomes less inviting and swimming can be dangerous, but the interior of the country — and the Buddhist monuments in town — are accessible year-round regardless of the weather.
Mangosteen season (roughly May to July) is a reason for some visitors to visit during the wetter months. The fruit is considered to be at its best just after the first rains, and Kalutara's roadside stalls are among the best places in the country to buy it.