Ko Samui has two seasons, sunny and rainy. The best time to visit the island is without a doubt from December through June. Coming in the off months means heavy rains and potentially torrential storms.

Ko Samui weather can become incredibly hot; those affected by weather should consider what month is best for them. December to February is traditionally the best time to go and weather averages around 86 degrees. March through June remains high-season but temperatures climb regularly to 95 degrees. Plenty of sunshine and very little rain exists during this period. October and November are the rainiest months, however, compared to main land Thailand, Ko Samui does not suffer nearly as much rain  (as opposed to the Andaman coast side of Thailand) during what's considered the "rainy season". People who visit Koh Samui have differing opinions and are affected differently by the weather. Somebody has written above that temperatures in the hottest period reach around 95 fahrenheit , 35c is the more common. If you stay away from the "wet season " which is around mid October to mid December you, on more occasions than not, will be ok. Visiting Koh Samui you should always expect some sort of rain due to the tropical climate

There are several fabulous festivals that will greatly highlight any visit to Ko Samui. One of the biggest events is the Thai New Year or " Songkran" on April 13. The word comes from Sanskrit meaning "new year" and the holiday itself was probably brought in from India as much as 2,500 years ago to Thailand and brought into Ko Samui later in its existence with the country. On Songkran day, people everywhere douse each other with water, if this doesn't sound fun to you, the parades and feasts taking place should make up for it.

The Ko Samui Music Festival is a grand celebration and one of Asia's largest music fests. Although, tourists should carefully consider coming in for the festival as it takes place September through October.