Bonn has a mild Rhineland climate. Summers are warm without being oppressive, winters are grey but rarely harsh.
May through September is peak season, with long days, outdoor dining, and a full calendar of events. The Rhein in Flammen fireworks display in early May is a spectacular start to the season -- boats line the river and street vendors set up along the banks. The Beethovenfest in September and October draws international orchestras and soloists for several weeks of concerts across the city.
Summer concerts in the courtyard of the Poppelsdorfer Schloss are a local favourite, running most evenings through the warm months.
Winter is quieter and cheaper. The Christmas markets in Bonn and nearby Siegburg are worth timing a visit around if you are in the region in December. Snow is possible but not guaranteed, and most indoor attractions stay open year-round.