Zagreb Fair (Zagrebački velesajam) is Zagreb's main exhibition and convention complex, hosting more than 25 trade fairs and events each year, including the long-running Interliber book fair.
Operating since 1946 (with trade-fair roots in the city dating back centuries), Zagreb Fair's pavilions host everything from consumer expos to international business conventions.
For the current calendar of fairs and events, check the official Zagreb Fair website linked above.