The south-west monsoon, sometimes known as the Yala monsoon, brings rain to Sri Lanka's south and west coasts in May. Galle and the beaches are likely to experience heavy bursts of rain, and the seas lose their tropical allure, but especially at the start of the month, there is still occasional sunshine in between. The capital, Colombo, can also expect a dousing.
No matter. Sri Lanka weather in May does look kindly on some. You can find the best weather on the east coast. Rain is slight and temperatures approach 30C so that is where the wise beach lover should head. The Cultural Triangle, dry and hot, is a perfect stopping-off point.
The eastern end of the south coast, in Tangalle and beyond, also has some of the lowest annual rainfall in Sri Lanka so it is rarely out of season.
May in Sri Lanka's hill country, especially in the south-west of the hills, can also be a wet month although Kandy and the northern hills still largely escapes the wet and can expect temperatures of 32C. That makes it easy to include some hill-country scenery in your holiday itinerary.