Bonn Summer Festival
May to late October
Bonn
You know a city is dedicated to celebration when it’s summer festival lasts not days, not weeks, but five months. The Bonn Summer Festival, or "Bonner Summer," is the bustling city’s main summer festival. The festival is open air and free. Its over 100 events focus mainly on celebrating cultural diversity, with concerts, performances, fireworks and other activities taking place all around the city. One of the festival’s most interesting offerings is a program of silent films, scored by live music, that are screened in the university courtyard.
http://www.bonn.de/
Also:
The Rhine in Flames fireworks festival, particularly in Koblenz (May-September).
www.rhein-in-flammen.de/
Red Wine Festival in Rudesheim (mid- to late May).
http://www.rudesheim.de