Sri Lanka has two monsoon systems that affect different coasts at different times. The southwest monsoon typically influences the south and west from roughly May to September, while the northeast monsoon affects the east from around October to January.
That split is why travelers can almost always find a dry-ish coastline. Pair it with realistic drive times and you avoid the classic mistake of chasing sunshine across the whole island in one week.
Use this as a planning lens, not a rigid rule: microclimates, Poya holidays and local festivals still shape day-to-day decisions.