The federal censuses can be one of the most valuable tools used in gathering genealogical data. But even the most experienced genealogists may not be familiar with some of the more obscure aspects of the census. One of these is called the "census day".
Beginning in 1790, each enabling law authorized by Congress specified a census day for gathering the information from every household in America. The census day was NOT the day the enumerator came to the household, it was the day for which all the statistics of the census were collected as dictated by Congress. The actual instructions given to the U.S. Marshals for taking the 1820 census reads:
".... all the questions refer to the day when the enumeration is to commence; the first Monday in August next. Your assistants will thereby understand that they are to insert in their returns all the persons belonging to the family on the first Monday in August, even those who may be deceased at the time when they take the account; and, on the other hand, that they will not include in it, infants born after that day."
Similar instructions have been given for every census since 1790. The table below indicates the census day and the time allowed to complete the census for a given year:
| Census Year | Census Day | Time Allowed |
| 1790 | 2 August | 9 months |
| 1800 | 4 August | 9 months |
| 1810 | 6 August | 10 months |
| 1820 | 7 August | 13 months |
| 1830 | 1 June | 12 months |
| 1840 | 1 June | 18 months |
| 1850 | 1 June | 5 months |
| 1860 | 1 June | 5 months |
| 1870 | 1 June | 5 months |
| 1880 | 1 June | 1 month |
| 1890 | 1 June | 1 month |
| 1900 | 1 June | 1 month |
| 1910 | 15 April | 1 month |
| 1920 | 1 January | 1 month |
From the table above, it's important to note that the census day changed from the first Monday in August in 1820 to the first day in June in 1830. If you are researching families enumerated in these censuses, you might want to look at these families again. Since there is a two month difference in the census day for these censuses, looking at these families again could reveal some surprising results.
For example, if a person were born between 1 June 1820 and 7 August 1820, that child would appear in the 1820 census in the "under 10" category. But in 1830, that same person would appear in the "of 5 and under 10" category rather than the "of 10 and under 15" category, since the person had not yet turned 10. The age category for anyone born between June 1 and August 7 of any year might be affected by this census day change. And, revisiting the census data for these people could give valuable clues as to their actual birth dates.
It's important also to note that a person may be in the census even though you have evidence to indicate that that person was deceased at the time of the enumeration. If that person were alive on the census day, they would be included in the enumeration. And, likewise, if a child was born after the census day, they would not be included in the enumeration.
In a couple of cases, there have been census days assigned to certain states that were different than the rest of the country. When Vermont entered the Union as the 14th state in1791, the 1790 census was already underway. So, Vermont's 1790 census was taken with a census day of the first Monday in April 1791 with five months allowed to complete the census. And Utah, which became a territory in September 1851 had its 1850 census taken with a census day of 1 April 1851.
