Short answer
Two windows: April to June, when the rivers are full and the karst hills are at their greenest, and September to October, when the air clears, the rice terraces turn gold, and the light is best for photography. October is the single best month for most visitors.
Season by season
Spring (March–May). March is cool and damp, with the hills often wrapped in mist — which, for a landscape of limestone peaks, is not a bad thing. April and May bring warmth and reliable river levels. This is the season for the Li River: the water is high enough for the full cruise route and the banks are bright green.
Summer (June–August). Hot and wet, with heavy rain in June and July and the occasional typhoon remnant pushing cloud in from the south. The scenery is lush and the rivers are at their fullest, but humidity is high and afternoon thunderstorms are common. The terraces at Longji are flooded and mirror-like in late May and June, which is a reason to come despite the weather.
Autumn (September–November). The best of the year. September is warm and clear, October is dry with excellent visibility, and the rice at Longji turns gold from late September before the harvest in October. November cools but stays bright and quiet.
Winter (December–February). Cool, damp and often foggy, with temperatures rarely below 5°C. River levels drop and the Li River cruise may run a shortened route from Yangdi rather than Guilin. In exchange, you get the karst hills in mist, almost no crowds, and the cheapest hotel rates of the year.
What the season changes in practice
The Li River cruise route depends on water level: in the dry months the boats sometimes start downstream at Yangdi instead of Guilin, which shortens the trip but also skips the first stretch. Ask before booking.
The Longji terraces have two peak looks — flooded and mirror-like in late May and June, golden in late September and October — and are bare earth or snow in between. If the terraces are the reason for your trip, plan around those weeks.
Dates to avoid
The first week of October combines the best weather with the worst crowds of the year: the river boats and the terraces both sell out, and hotel prices in Yangshuo triple. If you can shift to the second week of October, you get the same light with half the people.