Hamburger Dom is the large seasonal fair held on the Heiligengeistfeld near St. Pauli. It runs three times a year, in spring, summer, and winter, filling the fairground with rides, food stalls, games, and crowds.
It is one of Germany's biggest fairs and is worth planning around if your visit overlaps with its dates. The location is central, so it is easy to combine with St. Pauli, Planten un Blomen, or the Reeperbahn.