COOL MONTHS:
October - Mid March with temperatures ranging from 25 - 30 degrees celsius.

Koh Samui has a quite a short rainy period. The official southeast monsoon season starts in November and ends around early January, but it's difficult to predict nowadays. But even during the monsoon season, there are only a few days when it rains all day long. During the wet period, usually it will rain for a bit then the sun comes out for a few hours. The temperature is very pleasant with the northeasterly winds to cool off sunny days. Some days in December and January can be chilly because of the wind so bring a light jacket when traveling during this period.

HOT MONTHS:
Mid March - June with temperatures ranging drom 28 - 36 degrees celsius

The hottest period of the year is mid April and that's why the Water Festival is held on this month. With the northeasterly winds dying down, the temperature starts to rise and whatever breeze there is, usually is very gentle. Rain can be expected any day, but when it does rain it's usually only for a couple of hours unless there's a tropical storm or low depression in the area. Expect some rain around Mid May to June as a result of the effects of the southwest monsoon from the Andaman Sea.

WARM MONTHS:
July - October with temperatures ranging from 28 - 30 degrees celsius

After the effects of the southwest monsoon, the temperature drops to a more comfortable level where one can enjoy balmy days.

Check these links for weather conditions in this area:

They're not necessarily accurate because weather conditions in Koh Samui can vary from village to village even only a few kilometers apart, but it should give you a general of weather conditions.