Thailand has three distinct seasons: dry, hot and wet. Thailand has a large geographical spread from North to South and has many variations from the mountains around Chiang Mai to the tropical islands of the Andaman Sea and the Gulf of Thailand. Koh Samui weather is therefore different than other areas of Thailand.
Dry Season
Typically mid December through March when the weather is usually sunny and 30C/85F.
Hot Season
This season starts in April and lasts until mid to late September. The tropical sun is typically brighter and hotter during this period.
Wet Season
Koh Samui is a tropical island and you can expect some rain at anytime in any month. However, the wet season is October through to mid December, though it is still warm and there are generally many sunny days.
For forecasts and general climate history, good sources are usually weatheronline.co.uk and kohsamui.org