The south-west monsoon sets in during May. It puts wind and rain on the south and west, and it turns the same airflow into an offshore on the east coast, so Arugam Bay, Whiskey Point, Peanut Farm and Elephant Rock come alive over a few weeks, in that order and not all at once.
Understand the geography before you book, because it is the part people get wrong. Arugam Bay is about 320 km from Colombo and the road goes through the middle of the island rather than along the shore: six to seven hours, with published estimates stretching to nine. It is a travel day. Do not book a flight that lands in the evening. Once there, the four points are spread over about twelve kilometres, so a tuk-tuk or a scooter is part of the trip rather than an optional extra, and the camps run the shuttles.
The waves are a different proposition too. The south is a chain of reefs you walk between; the east is a run of long right-hand points you drive between. And the east is limited by size rather than by wind, which is the single most useful thing to carry with you: at Peanut Farm even in its best month, six days in a hundred are blown out and fifty-nine are too small. In May the season is still filling in, so you watch the swell and let the wind take care of itself.
Two coasts are at their own peak in May, Nicaragua at 29°C and South Africa at 18, and Sri Lanka is not one of them: it is good rather than excellent, with the east still building. That is the case for coming anyway. The town runs on its season and closes after it, so May is Arugam Bay waking up, with the main street opening rather than open, prices low, and the points empty for another six weeks.

