Census Day is the day as of which a census

Census

A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common...

 aims to determine where each citizen is.

United States

In the United States' recent censuses, Census Day has been April 1. However, it used to be in August, as per these instructions given to U.S. Marshals: "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."