Many people find the summer season far too hot, whilst newcomers to Thailand find the monsoon season humid and sticky. Generally, it only rains for two or three hours a day during the monsoon season, usually in the early morning and then again around 5 pm.
Most people prefer to visit Thailand between November and February, when the weather is dry and relatively cool and the beaches are at their best. This is also when most of Thailand's festivals take place.
The peak tourist season is from November to late March, with a secondary peak season in July and August. However, this is also naturally when accommodation is the most expensive. People wishing to save money and avoid the crowds would do well to visit during September and October, when the fear of rain keeps most tourists away.
Thailand's size and location naturally means that different areas of the country are better to visit at different times of the year.
Central Thailand and the East
Often roasting during the summer (40 degress C is not unusual), it rarely gets cold in the capital and surrounding area. If you want to beat the heat and the often torrential rain of the monsoon season, it is best to visit between November and February, with the coolest weather usually in January.
Northern Thailand and the Northeast
This region is quite a bit cooler than the rest of Thailand. Although you may need a jacket, November-February is the best time to visit as it is a good time to see the flowers and spectacular scenery. If in Thailand during April, the north is a little cooler, so this region makes a good summer retreat.
This region is probably best avoided during the monsoon season, especially if you are planning to visit the islands. The island regions often experience heavy storms during the monsoon season, and it can be difficult to travel by boat. Once again, November to April is probably the best time to visit this area.