Harry's Weather : -

December to February (main season) it can get up to 30°C (86°F). Sometimes brief 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 (114F), rarely raining. You will enjoy to have a bungalow with air conditioning !

July to September (main season) is hot, but some days it also rains. August is called the Italian season (you can guess why).

June, October, November are rainy seasons, but it does not generally rain every day. In contrast to other regions of Thailand, Samui does not really have a well defined rainy season but sometimes there are sunless periods of 2 or 3 days, especially during November (monsoon).