About the climate and weather in Thailand many errors are spread. The main reason for this is that there are three different climate zones are in Thailand:
the North, the West Coast and the East Coast. In all three areas have completely different seasons. Ie. A month of is completely rained out on the West Coast, on the East Coast can be beautiful, etc.

Still the most tourists in Thailand visit the west coast in the area of Phuket and Koh Phi Phi. In the 1990s, this was almost the only area that was developed for tourism. Therefore, most travel guides and storries from tourists are from that area.

May is for example one of the best months here on Koh Tao - but almost everyone thinks May is rainy and cold.

If there is a problem on Koh Tao, then it is water scarcity. We locals always say: "Koh Tao is so small, it is overlooked by the rain." Even in months with lots of rainfall on the east coast there is on Koh Tao significantly less rain than on Koh Phangan or even Koh Samui.

Here is an overview of the seasons of the year on Koh Tao:
(Of course, everything always +/- 2 weeks)

There is only one real rainy season on the east coast and that was the last years end November/ beginning December.
After that, it goes in the dry season, but the transition usually takes place by the middle/ end of January.
The warmest months are March and April.
In May, it is pleasantly warm and little rain.
In June, the weather on the west coast is bad, we get on Koh Tao mostly just some wind and some rain showers.
In mid-July the weather stabilizes again and normally the entire summer is very nice.
Even October and the beginning of November is usually very sunny.
Then the year starts again from the beginning :-)

Click for large picture.