The dry, cool months from November through February are by far the most comfortable. Daytime temperatures sit around 25-30°C, evenings are pleasantly cool, and the humidity drops. This is also when the That Luang Festival, the largest religious event in Laos, lights up the city in mid- to late November — three days of processions, a candlelit walk around Pha That Luang, and a sea of food and craft stalls on the surrounding fairground.
March to May is brutally hot. Temperatures regularly top 35°C, the air is thick, and the haze from agricultural burning across the Mekong can sit over the city for weeks. Avoid these months if you can.
The rainy season runs from May to October. Afternoon thunderstorms are the rule, but they are short and dramatic rather than endless drizzle, and the countryside turns lush green. The Boun Bang Fai (Rocket Festival) in May, when villagers build and launch homemade rockets to prod the heavens into delivering rain, is worth seeking out in the surrounding province.
In April, Pi Mai (Lao New Year) turns the whole city into a multi-day water fight. The streets fill with revellers throwing buckets of water and smearing each other with white paste. It is enormous fun if you embrace it, miserable if you try to dodge it.