Vienna works in every season — there is no truly bad time — but each has its character.
Spring (April–May) is the sweet spot: warm days, cool evenings, lilacs in the parks, and the city not yet overrun. The Burgtheater and Staatsoper are still in season. The Wiener Festwochen, the city's main contemporary arts festival, runs through May and into June.
Summer (June–August) is hot — often above 30 °C in July and August — and the opera houses are closed for the summer break. The compensation is the outdoor life: open-air cinema at Rathausplatz, beach bars on the Donaukanal, swimming on the Donauinsel and Alte Donau, and the Heuriger wine taverns spilling into their gardens.
Autumn (September–October) brings cool, clear weather and the new wine (Sturm) at the Heurigen. The opera season resumes. The leaves in the Vienna Woods turn vividly red.
Winter (December–February) is cold but theatrical. The Christmas markets from mid-November to 23 December are the headline draw; New Year and the famous ball season in January and February are the locals' favourite time. Skies are often grey, temperatures regularly below freezing. Pack accordingly.
The Vienna Ball season (January–February), the Opernball in particular, is a unique spectacle if you can secure a ticket or simply watch the arrivals at the Staatsoper.