November to February is the high season — the dry, cool months when the weather is reliably sunny, the sea calm, and the trade winds bring decent windsurfing conditions. Expect higher hotel prices and busier beaches; book ahead at New Year.
March to May is hot and humid but still mostly dry; the sea is at its warmest. June to October is the rainy season, with heavy afternoon showers and a generally murkier sea. It is also low season — half-price hotels, empty restaurants, and you may still get a week without significant rain.
The annual Jomtien International Fireworks Festival in late November is a good reason to be in town. Songkran (Thai New Year, April 13–15) is celebrated with the country's biggest water fight; quieter here than in Pattaya, but still wet.