The best overall time for visiting most of Thailand vis a vis climate is between November and February - during these months it rains least and is not too hot. The south is best visited when the rest of Thailand is miserably hot (March to May), and the north is best from mid-November to early December or when it starts warming up again in February. If you're spending time in Bangkok, be prepared to roast in April and do some wading in October - probably the worst two months, weather-wise, in the capital. The peak tourist months are December and August, and the least crowded months are May, June and September.
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