Passau is pleasant from April to October. Summer brings the most visitors; the old town can feel crowded in July and August, particularly during the Danube cruise season. The European Weeks festival (Europäische Wochen), held from late June to late July, is an international music and arts festival that fills the city with concerts, including outdoor performances in the old town.
Spring and early autumn are the best times to visit — mild temperatures, manageable crowds, and the chance of good light for the views from Veste Oberhaus. Winter is cold and quiet; the Christmas market in December transforms the old town squares. Be aware that Passau floods regularly in spring snowmelt season — the Danube can rise quickly, affecting riverside paths and occasionally the lower parts of the old town.