Peak season in Hikkaduwa is between November and April, with January, February and March being the very best months to visit for surfing and diving.
The majority of the rain usually falls around May and around October in South Western Sri Lanka although, being an island, it could rain any time during the year. There are only a few days in the year when it rains solidly - otherwise, it just comes down in a short sharp downpour. Lots of people, including me, really like the rainy months because the air is clearer and cooler and everything looks more tropical and lush. It's actually fun to sit under the veranda and watch a tropical downpour!