The climate of Ko Samui is a tropical savanna kinds with heavy rainfall almost all around the year. The climate is usually humid and warm. The temperature is usually around 27 – 28 degrees Celsius throughout the year.
If you want to enjoy at the beach then plan a visit in the dry and relatively cooler months of December to April. This is a best time to visit Ko Samui, the only drawback being the large number of other tourists.