The Dayton Hamvention is an amateur radio convention (or hamfest), generally considered to be the largest hamfest in North America. It is held each May at the Hara Arena in Trotwood. The Hamvention offers forums, exhibit space and a flea market and usually claims to have over 20,000 visitors (verified by the Amateur Radio Relay League). 

The event is sponsored by the Dayton Amateur Radio Association.

http://www.hamvention.org/