Peak season to visit Sri Lanka is December to March when the country sees little to no rain and the temperatures are warm and pleasant throughout.
Shoulder Season: May to September is when the temperature ranges from hot and dry to wet and agreeable. From May onwards, some parts of the country see intermittent showers, bringing temperatures down.
Off-season: April and October to November. In April, early mornings and evenings are a good time to sightsee, though you should stay inside in between. October to November is when the country welcomes the heavier of its two monsoon, which affects the whole country.