November to February is the best time: cooler and drier, good for both Bangkok and the west-coast beaches.
- March–May: very hot; mid-April brings the Songkran water festival — great fun but crowded, with higher room rates.
- May–October: rainy season with afternoon showers and lower prices; the Andaman coast (Phuket, Krabi) has rough seas.
- Gulf islands (Koh Samui) get most rain in October–December, the opposite of Phuket.
Note: for Songkran, bring a waterproof pouch for your phone and passport.