Koh Samui's location in the Gulf of Thailand sees the island enjoy fine weather, meaning less rain, for probably more months of a given year than it's counter parts in the Andaman Ocean. The island does see it's annual tropical downpours, but it is unlikely you will ever be stuck inside your hotel room for more than a few hours due to rain any time of the year. In Phuket for example, the rainy season lasts from beginning May to end October. In Samui most rain is experienced during May and June. July and August sees much less rain than the preceding two months and September, October and November will again see an increase in precipitation. Rain usually comes in the form of short, heavy burst of thunder showers which lasts an hour or two in the morning and again late afternoon or evening. During this time you can still expect temperatures to average 28 to 29 degrees Celsius. December until February is dry with little rain if any, temperatures averaging 32 degrees Celsius and of course the ever present humidity to keep in mind. March and April are hot months in Samui, little rain and temperatures soaring to 40 degrees Celsius. The fact is, booking accommodation in Samui is popular all year round and securing your room well in advance to ensure availability is always recommended due to the island's popularity. Hotel and resort prices during the rainy season will see you pay mentionable less for your accommodation and special deals like free night promotions and early bird prices during these months makes things even cheaper. So don't wait until it's too late and book now !