December is one of the most pleasant times of year to be in Thailand.
The West coast of Thailand, which includes Phuket, Krabi, Khao Lak, Phi Phi Island and Koh Yao usually experience long days of sunshine and little rain, although an afternoon downpour is always possible.
Bangkok and Chiang Mai are also in the dry season and Koh Samet is also a great choice for a beach holiday at this time of year, being the driest region of Thailand.
Rainfall in Koh Samui is also much lower in December than in November and December is a good month to consider a beach holiday in Koh Samui or Koh Phangan.
Temperatures in the mountainous areas of the north are lower between November and March and it can be decidedly chilly in the morning at altitude.