Bad Gastein has two proper seasons. Winter (December–March) is ski season: the town is part of the large Ski Amade network, sharing 860 kilometres of marked runs with Schladming, Filzmoos, and others. Snow is reliable at altitude; Bad Gastein's own ski area tops out around 2,700 metres. The thermal baths are especially good in cold weather — soaking outdoors in warm water while snow falls is a genuine alpine pleasure.
Summer (June–September) is hiking season. The valley is spectacularly beautiful and the trails are well-marked, ranging from valley walks to serious ridge routes. Temperatures are mild and reliably cooler than the Austrian lowlands. The baths are open year-round and equally good in summer.
April–May and October–November are the shoulder seasons when many facilities close or reduce hours. The town is quietest then — fine for a cheap visit, but check which baths and restaurants are open before you go.