The best time to visit Thailand is from Nov to Feb when the weather is dry and the temperatures are not too hot. During the winter, the temperatures in the far North can drop down to freezing during the evenings.
From Mar to Jun the heat returns with temperatures soaring to over 40ºC during the day with minimal rain and low humidity.
The rainy season begins toward the end of Jun and continues until Oct, generally consisting of a short but sharp downpour in the afternoon.
The Southern beaches are faced with two weather systems which bring the tropical rains at different times of the year, so it's possible to enjoy Thailand's beaches year-round. From Jan to Sep the East coast is sunny and dry, before moving to the West coast from Oct to Apr.