May to September is the main visiting season, when the ferries and the Flåmsbana run at full frequency, the Otternes farm is open, and the waterfalls are at their most impressive from the spring snowmelt. July and August are the busiest months — cruise ships regularly dock at Flåm, and the village can feel overwhelmed midday. Early morning or late evening in summer is a different experience entirely.
Autumn (September–October) is excellent: the deciduous trees along the valley turn gold, the crowds thin out, and the light on the fjord takes on a colder, more dramatic quality. Winter services are reduced, but the Flåmsbana continues to run, and the snow-covered valley is beautiful if you're willing to accept shorter daylight hours and limited services.