Tropical Koh Samui has perennially good conditions, save a rainy season that spans the last quarter of the year. Average year-round temperatures linger in the upper 80s. This means its robust tourist scene is almost always busy. The peak time for visitors, however, is from January to July, when seas are calm, skies are clear, and weather is mild. Expect beaches and attractions to be packed with visitors, especially on heavily developed Chaweng and Lamai beaches, and accommodation prices to be much less flexible than in less upscale Southeast Asian destinations.
The October to December low season sees a sharp increase in rainstorms, with November bringing particularly heavy rainfall. Think an average of 11 inches per month. During this time, the water can get cloudy, affecting visibility for diving and snorkeling, and beach conditions can get rough. That said, storms generally roll in only during afternoon, and the island still offers a lush, beautiful vacation backdrop, with inland rivers and waterfalls swelling with water. The rains also bring reduced crowds and lower prices for some accommodations.
In between the peak travel season and the wet season, the August and September shoulder season can offer slightly reduced crowds. This time of year also has occasional showers, and it can get quite hot during the day.
With a diverse local population that includes Chinese, Indian, and Malay influences, as well as a mix of Buddhists and Muslims, there are festivals and holidays year round. Songkran, the Thai New Year, falls in mid April. The holiday culminates gleefully when locals and tourists alike take to the streets in a massive water fight and douse each other with ice-cold water.