Bandung's altitude — 750 metres on the city floor, much higher in the surrounding hills — keeps things cooler than the rest of Java year-round. Daytime temperatures in the city centre sit between 22 and 28°C; the highland excursions are often noticeably cooler, particularly around Lembang and Ciwidey.
The dry season runs roughly May to September, with sunny days, cool nights, and the best photography light. The wet season runs October to April, with frequent afternoon downpours and the occasional cloud-locked day on Tangkuban Perahu. Bandung is at its prettiest just after a rain shower, when the volcanoes briefly emerge from cloud over the basin.
Weekend traffic from Jakarta is real and substantial. If you can, plan city sightseeing for Mondays to Thursdays. The big festival on the local calendar is the Asian-African Conference Commemoration in late April, which fills the pedestrianised stretch of Jalan Asia-Afrika with parades and exhibitions.