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December to March are generally the best time to travel to Thailand - but as with any tropic holiday resort, a brief and ludicrous deluge is possible even on the most sunny sunday. Thailand's Gulf of Thailand's Thai archipelagos - Koh Samui, Koh Phangan, Koh Tao - are prone to less rains than the remainder of the land (even in the wet season) and are at their best from the end of January to mid-March.
Jan is the coolest months - ideal for the post-Christmas and NYE parties mayhem. It is the wettest months of February, one of the most sunshiny and still tolerable in the sun. And March is also sunshiny and withered. On Thailand's western coastline and the Andaman Sea - where the Thai isles of Phuket, Krabi, Koh Lanta and Koh Phi Phi are located - the ideal place for a seaside vacation is a little early, in December and January, when cooler days, as well as the sun and minimal precipitation come together to provide the best conditions for a seaside leisure.
Bangkok is wet almost all the year round (complaints vary from "high" to "extreme" all year round), but the best season to come to Thailand's capitol is December and January, when it is arid and a relatively tolerable daytime and nighttime mean of 31-32ºC (the coldest Bangkok has ever had).
If not to Bangkok: Please don't go to April and September. The beautiful city of Chiang Mai and Pai, in the colder northern part of Thailand, have a much more comfortable climate than Bangkok and the southern part. From December to February for nice sunshine nights (around 29ºC) and colder nights (the temperatures drop to 13ºC - and even lower in the highland, so take out your hot clothing when you hike).
When the monsoon comes down from all sides, the period from May to October is the rainfall.