When to go?

Climate of Thailand
Thailand is largely tropical, so it's hot and humid all year around with temperatures in the 28-35°C range (82-95°F), a degree of relief provided only in the mountains in the far north of Thailand. The careful observer will, however, note three seasons:
Cool: From November to the end of February, it doesn't rain much and temperatures are at their lowest, although you will barely notice the difference in the south and will only need to pack a sweater if hiking in the northern mountains, where temperatures can fall as low as 5°C. This is the most popular time to visit and, especially around Christmas and New Year's, finding flights and accommodation can be expensive and difficult.
Hot: From March to June, Thailand swelters in temperatures as high as 40°C (104°F). Pleasant enough when sitting on the beach with a drink in hand, but not the best time of year to go temple-tramping in Bangkok.
Rainy: From July to October, although it only really gets underway in September, tropical monsoons hit most of the country. This doesn't mean it rains non-stop, but when it does it pours and flooding is not uncommon. There are local deviations to these general patterns. In particular, the south-east coast of Thailand (including Ko Samui) has the rains reversed, with the peak season being May-October and the rainy off season in November-February.

Check update 5 day forecast in BBC Weather


When to go?
Most visitors head to Thailand during the relatively cool and dry season from December to February, when rainfall is scarce and the temperature around a tolerable 84-90°F(28-32°C). (If you’re traveling in the hills, temperatures can drop sharply in the evenings, so take a sweater.) The drawback is that popular destinations are crowded and hotel rooms can be in short supply. Prices tend to be higher during the tourist season and internal transportation can be difficult to secure. March to May or June is the hottest time to visit temperatures may reach 100°F (38°C), particularly in the plains of the Northeast, and the high level of humidity (around 80 percent) can make travel uncomfortable and oppressive.The wet season runs from June to November, and this may be the finest period to enjoy the charms of the country without the hordes of tourists. These months constitute the “rainy” season, when a dose of precipitation is almost guaranteed most afternoons. The remainder of the day will generally be dry, if overcast. Major monsoons, which can last several days, are relatively rare. Another advantage of travel at this time of year is that the countryside is vibrantly green with flooded rice fields and blooming trees. Vacancy rates are high at most hotels, and it is easier to make reservations with local transportation.