May, June, and September are the sweet spot — warm days, long evenings, mountains still snow-capped from the old town, and tourist crowds well below midsummer peak.
July and August belong to the Salzburg Festival, when the city more than doubles in temporary residents and every other person in the old town is in evening dress on their way to or from a concert. Hotel prices triple. Even if you have no ticket, the atmosphere is worth experiencing once.
The Christmas markets on Residenzplatz and Mirabellplatz, running from late November to 26 December, are exceptionally good — a mix of trinkets, Gluhwein, and choirs in the cold. January and February are very quiet and cold but cheap; the surrounding ski resorts of Werfenweng and the Salzkammergut take over.