The best months are May to September, in the dry season — river trips into Sebangau are easier, the trails firmer, and the orangutan-spotting more rewarding. October to April is the wet season; rain is heavy and frequent, and in particularly dry years (notably 2015 and 2019) the surrounding peat-swamp forests have burned, leaving the city under a haze of acrid smoke for weeks. Check air-quality reports for the time of year you're considering.