Austria has two distinct peak seasons, and both are excellent. Winter runs from December through March and is all about skiing, snowboarding, and Christmas markets. The big ski resorts in Tyrol, Vorarlberg, and Salzburgerland are buzzing from late December through February — that's also when prices are highest. Christmas markets open in late November across Vienna, Salzburg, Graz, and Innsbruck, and they're genuinely atmospheric, not just tourist traps. If you're skiing on a budget, January after the holidays and March offer better deals and fewer crowds.
Summer, from June through September, is hiking season, lake season, and festival season. The Alps are spectacular in July and August, with wildflower meadows, clear mountain lakes, and long daylight hours. This is when the Salzkammergut lakes around Hallstatt are at their best, and when Alpine huts are open for multi-day treks. The Salzburg Festival runs from mid-July through August and is one of the world's great classical music and opera events — book well ahead if you want to attend. Vienna is a year-round city, but summer brings open-air cinemas, Danube Island beaches, and a more relaxed pace.
The shoulder months are underrated. May and early June bring warm weather, green valleys, and smaller crowds. September and October are gorgeous — the trees turn, the new wine (Sturm) arrives at the Heurigen, and you get crisp sunny days without the summer heat. November and early April can be grey and in-between: too late or too early for skiing in lower resorts, too cool for comfortable hiking, though Vienna and the cities don't mind a shoulder season at all.