From 1790 to 1820, the censuses were conducted as of the first Monday in August (August 2, 1790; August 4, 1800; August 6, 1810; August 7, 1820); the 1830-1880 and 1900 censuses were as of June 1; the 1890 census was as of June 2; April 15 was Census Day in 1910; and the 1920 census was as of January 1. Since 1930, Census Day has been April 1.

Puerto Rico was first enumerated by the U.S. Census Bureau for the U.S. War Department in 1899. Puerto Rico has been included as part of the decennial census since 1910. The Census Bureau conducted the first economic census in Puerto Rico in 1909. For more information about the censuses in Puerto Rico and other U.S. Territories, visit the Island Areas Web page.

The first census in the United States took place beginning on August 2, 1790. Although it took months to collect all the data from households, census takers were instructed to collect information as of August 2. For more information, see the 1790 overview page.

The Census Bureau first used statistical sampling methods during the 1937 test survey of unemployment (Enumerative Check Census). This survey estimated the scope of unemployment in the United States during the Great Depression and served as a "check" on a more inclusive, voluntary census effort attempting to measure the nation's unemployment rate.

From 1790 to 1820, the U.S. Marshals conducting the census only received instructions about what to ask. Each marshal supplied his own paper and used whatever method he chose to divide the paper into the columns needed to collect the required information. The method for recording the data was not standardized until 1830 when marshals received uniform printed schedules.

The Census Bureau began collecting housing data as part of the decennial census beginning in 1940.

Records from the 1790 to 1870 censuses were public record and were not protected. In fact, U.S. marshals would post the records in a central place in communities for public viewing. The Census Act of 1879 made census records confidential and included penalties for disclosing the data. In 1954, Congress codified the rules regulating the confidentiality of census data as Title 13, U.S. Code.