The best time to visit Koh Samui is from the end of December to April. Though there is not much variation in temperature there are high chances of rain throughout the year which is why Koh Samui is the most pleasant during these months.
The weather is mostly dry and warm, and it is the best time to visit the beaches. The average temperature through the year varies from 26-27 degree Celsius but can cross 30 degrees during the hot summer months of May and June. October to late December is when the tropical rain lashes down on the island and is regarded as the wettest months of the year. The wet months are not the best time for scuba diving or bathing, but there are plenty of activities that you can take recourse to.