Bangkok has three seasons: cool, hot, and rainy -- or as foreigners tend to experience them, hot, hotter, and hot and wet. November through February is the cool, dry season with temperatures in the mid-twenties to mid-thirties Celsius and the lowest humidity. This is the most popular time to visit.
From March to May, temperatures regularly hit 40 degrees and the humidity is punishing. Light, loose clothing, a hat, and plenty of water are essential. The rainy season runs from June to October, with October being the wettest month. The rain usually comes in dramatic afternoon downpours that clear quickly, so it is not as bad as it sounds.
Festivals can make a trip more memorable. Songkran (Thai New Year) in mid-April turns the entire city into a water fight -- brilliant fun if you do not mind getting soaked. The Lunar New Year in late January or February brings Chinatown to life. In November, the Loy Krathong festival sees thousands of candle-lit floats released onto the river, and the Royal Barges Procession is a rare and spectacular sight.