Sri Lanka vacations in September fall at the slow end of the rainy season. The prolonged monsoon rains are coming to an end. However, they are replaced by zenithal rains. They have a plus and a minus. Zenithal rains are not as prolonged as monsoon rains, but they come in the daytime, sometime after noon. But this hardly spoils your beach tour to Sri Lanka in September, as the rains end very quickly.