From November to Ash Wednesday: Carnival
The season officially starts on 11/11 at 11 hrs 11 minutes; from that time on, expect to hear about Carnival choice of prince and a number of other traditions and events. The Carnival high point starts six weeks before Easter, usually in February, the first event being "Weiberfastnacht" (Thursday) when women take over. Men should wear old ties as women will cut them off on that day, beware! "Rosenmontag", heart of the Carnival: you will see parades in various centres during the day, candies are thrown into the crowd. However, most of the Carnival festivities take place indoors in pubs, association halls, etc. If you are planning to go to one of those, don't forget to dress accordingly.
Cologne has a particularly large Carnival parade.
May: Rhein in Flammen
In May, look for the "Rhein in Flammen". Beautiful fireworks all along the river crowned by spectacular fireworks in the Rheinaue park. Entrance free. Great music programme.
May/June: Museum Mile Festival
In May/June, the Museum Mile Festival invites to a lot of attractions for all ages.
In July, the "Bierbörse", a festival/fair of beer will take place in the Rheinaue park. You will get a chance to try different kinds of beer (more than 700 varieties) from all over the world.
During the summer there are usually a number of other events and festivals at Rheinaue park, such as an international festival, a balloon festival, and a very large flea market on every third Saturday of the month. There are also plenty of other second-hand, antique, pottery, etc. markets through the year in Bonn and surroundings. They are usually advertised in the local paper (Bonner General-Anzeiger), in the Saturday edition under the section Boulevard.
September: Pützchens Markt
Pützchens Markt at Beuel. Enormous fun fair held in early September. One of the biggest in Germany!
September/October: Beethoven Festival
Every autumn, the Beethoven city of Bonn celebrates the Beethoven Festival (Beethovenfest). Come and see top international orchestras, respected ensembles, prominent soloists, and promising young artists. More information can be found here.
November: St. Martin's Day
In November, St. Martin's Day is celebrated. Children join parades in the streets with lanterns, sometimes following a man on horseback dressed as the Roman soldier, St Martin. The parade usually ends with a bonfire and children are given a "Weckmann" - a bread man.
December: Christmas Markets
Before Christmas you will find Christmas Markets in most town and city centres (Bonn centre, Bad Godesberg centre, several in Cologne, etc.). Siegburg has a unique medieval fair. It is recommended to visit it in the late afternoon when you can soak up the "medieval" atmosphere by the light of flaming torches.
An overview of all highlights in 2014 can be found here.