Climate & The Best Time for Traveling
Basically, Thailand has three seasons: a dry and hot season (from November until February), a very hot season (from March until the end of April the weather is still dry, but very hot, and doing sightseeing tours may be uncomfortable) and a wet and hot season (this is the monsoon season from May to the end of October).
December to February (main season) it is up to 30°C (86°F). Sometimes a short tropical rain, mostly in December. This is the best season for European people, because it is not too hot.
March to June it becomes very hot, up to 40°C (114°F), are rarely raining. You will enjoy to have a bungalow with air condition!
July to September (main season) is hot, but some days its raining. August is called the Italian season (you can guess why).
June, October, November are rainy seasons. But its not raining every day. In contrast to other regions of Thailand, Samui not really have a raining season. But sometimes there are sunless periods of 2 or 3 days.