Thailand's monsoons arrive around July and last into November (the 'rainy season'). They can be uncomfortably, unpredictably sticky. This is followed by a dry, cool period from November to mid-February, followed by much higher relative temperatures from March to June.

By far the best time to visit is from February to March when the weather is kind and the beaches are at their finest.

The peak seasons are August, November, December, February and March, with secondary peak months in January and July.

If your main objective is to avoid crowds and to take advantage of discounted rooms and low-season rates, you should consider traveling during the least crowded months (April, May, June, September and October).

On the other hand it's not difficult to leave the crowds behind, even during peak months, if you simply avoid some of the most popular destinations (e.g., Chiang Mai, islands and beaches). This is also the prime time for diving in terms of visibility and accessibility.