During the high season, from December to February, temperatures on Koh Samui can reach up to 30C (86F). Brief tropical rainshowers occur mostly in December. This is the best season for European people as the weather is comparatively cooler than the rest of the year. From March to June it can become very hot -- up to 40C (114F), with little or no rain. You will appreciate your air-conditioned accommodation during this time! The months of July to September are hot with occasional rain. These are also popular months for visitors. October and November see more rain than other months, though it generally doesn't rain every day. In contrast to other regions of Thailand, Samui does not really have a well-defined rainy season but sometimes there are sunless periods for two or three days, especially during the monsoon month of November. The weather is quite difficult to predict during this time, but being Samui, you're virtually assured of getting some great sunshine!