BSB is hot and humid year-round, with temperatures between 24-32°C and humidity that makes it feel hotter. There is no truly cool season, so come prepared to sweat.
The wettest months are October through January, when heavy afternoon downpours are common. These tend to be short and dramatic rather than all-day affairs -- an umbrella is enough. February and March are the driest months and generally the most pleasant time to visit.
If you want to visit Ulu Temburong National Park, the drier months (February-August) are preferable, as trails can be slippery and river levels unpredictable during the wet season.
During Ramadan, many restaurants and food stalls close during daylight hours, which limits your eating options significantly. The night market, however, becomes even more lively after sunset. The three-day Hari Raya Aidilfitri celebration at the end of Ramadan is a unique time to be in BSB -- the Sultan opens the Istana Nurul Iman to the public, and the city is festive and welcoming.
The Sultan's birthday (July 15) kicks off two weeks of parades, fireworks, and cultural performances. Chinese New Year (January/February) also brings festivity to parts of the city.