Visiting Mirissa Sri Lanka? Whale watching is a must do activity in Mirissa. Imagine being just feet away from the largest and most graceful mammals in the world. Mirissa is a hotspot for whale watching in Sri Lanka, with the best time for sightings being during the months of November to April. December, January and April are the peak months and the chances of spotting sperm whales and blue whales during these months is very high.