Great all year round...
As rainfalls shower different regions at different times of the year, one can go to Thailand all year round.
Thanks to the Thai-Birmanese Tenasserim range, rain comes at different times on the Western and Eastern coasts of the Malay peninsula.
Bangkok is an idyllic place to visit all year round – apart from April and May when it is very hot. From November to April, Southwestern Thailand (for instance Phuket and Khao Lak) offers sun and pleasant temperatures (around 30°C).
Koh Samui and Southeastern Thailand are pleasant places all year round, although short and strong showers these regions in November and December. But it is always warm since the sun is never far away.
In Hua Hin, beaches are partly flooded in August and September. Bathing is then restricted to swimming pools.