Thailand is understandably very popular in January as the weather is good throughout country throughout the month. It’s a particularly good month for touring as there is little chance of rain and for example northern Thailand will be enjoying cooler (avg 24°C) temperatures that are conductive to exploring on foot whilst the southern beaches, especially on the west coast, will be bathed in warm sunshine (high 20s°C). The sea round the west coast will be warm and calm, so perfect for diving around Phuket, Krabi, Phi Phi and Khao Lak.
On the east coast the wet season is drawing to a close and so expect a few showers in the early part of the month in for example Hua Hin or Koh Samui.