The best time of the year to visit Koh Samui is late December to April has excellent weather with a lot of sun. May to September there’s still plenty of sun, but you will get some rain – usually brief showers in the afternoon or night. 

October to November is Monsoon Season, and yes, it might rain heavily, it can be very windy, and storms are possible, but even during monsoon, it usually lasts only a few hours at a time. The air is nice and cool (tropical standards for cool), and nature is blooming because it gets enough water.