Thailand's climate is tropical. High in both temperature and humidity. Most people find it pleasant. The average temperature is about 30 degrees Celsius, or 86 degrees Fahrenheit. When it is getting too hot, just enter a air conditioned room or shop for a while and you will "refill your batteries". Remember to drink a lot of water. Even if it is more or less "summer" here year round, you could say that Thailand and Koh Samui has three seasons:
Dry Season (December - February) The dry season on Samui. Not much raining, but it can be a some refreshing wind or breeze on these islands then. Great for bodysurfing. The sun is shining a lot on us at this period of the year!
Hot Season (March - August) The weather is still dry in this time, and rather hot. In May the raining may increase a little. This is a great period to visit Samui if you can stand 30-35 degrees Celsius daytime. The sunshine is still dominating the day time, almost as much as in the Dry Season.
Rain Season (September - November) Also known as the monsoon season. It's still hot and normally, there are many sunny days anyway. Sometimes you might find yourself in a refreshing monsoon shower for a moment. The rain showers often last just for a short while though and as these island's water supplies are finite, the water is needed.