Sri Lanka is a Tropical Country because it’s situated near the Equator. Although it’s a small island it’s weather is rather complicated. The main Southwest (Yala) monsoon brings rain to the west and southwest coasts and hill country from April/May to September (wettest from April to June). The less severe Northeast (Maha) monsoon hits the east coast from November to March (wettest from November to December); there’s also a inter-monsoonal period of unsettled weather preceding the ‘Maha’ monsoon in October and November during which heavy rainfall and thunderstorms can occur anywhere across the island.
So the best time to Visit West, South and Hill Country is from December to March whilst the best time to Visit North and East Coast is from April/May to September. So this Paradise island is all Year around destination because one region has good weather and will cater the needs of a tourist.
Since the island is close to the Equator, the temperatures remain fairly stable at around 25 – 30 °C during the day and 18 – 25 °C during nights. But hill country is rather colder than other parts of the country.
What I recommend is that you do your research before travelling to Sri Lanka to avoid any inconvenience. Or best way is to consult a travel agent.