While the north and east coast go through their monsoon season, the south and west is dry and perfect for travel, with cool temperatures and blue skies. Ditto with the green and hilly interior, for treks and bike rides through the tea estates and safaris in wildlife parks. Christmas and New Year holidays are super-busy so book in advance.
April still has dry weather, but temperatures may rise in anticipation of the SW Monsoon, which hits Sri Lanka around mid-May onwards. August November is when tourism picks up again after a wet and wild spell of the SW monsoon.
The monsoon is here, pretty much hammering the south and west coast, leaving the north and east dry and perfect for travel. The hilly interior is rainy but good for nature treks if you don't mind getting wet. Waterfalls are in full flow and the greenery is stunning.