Series: M33   Rolls: 1 - 142 1820 Facts 1. Fourth U.S. decennial population census. 2. U.S. population: 9.6 million     (8.1 million free, 1.5 million slave) 3. Census conducted by U.S. marshals under the     direction of the Secretary of State. 4. Census day: 1st Monday in August (7 August 1820). 5. Mandated copies: One set, sent to the federal district     court. Same copy ordered sent to the     U.S. Secretary of State in 1830.