The Salzkammergut is most visited in July and August, when the lakes are warm enough for swimming and the mountain trails are all accessible. Hallstatt in particular becomes very crowded — tour buses arrive from Salzburg and Vienna from 9am daily — and the experience of the village in high summer can be frustrating. The same months bring the best weather: reliable sunshine, warm lake water, and long evenings.
May and June are good months: the mountains are still partially snow-covered and the flowers are out in the meadows, but the crowds are lighter and prices lower. September is often the best month of all — warm enough for the lake but quieter than summer, with the larch trees turning gold on the lower slopes and the apple orchards bending under their fruit.
Winter brings skiing to the Dachstein and a quieter, genuinely Austrian atmosphere to the towns. Bad Ischl and the larger villages have Advent markets in December; Hallstatt's market is particularly atmospheric, lit by lanterns and with views over the frozen lake. The caves are closed in winter; many hotels and restaurants close in January and February.