Article 1, Section 2, of the United States Constitution requires that a Census be taken every 10 years for the purpose of apportioning the United States House of Representatives. U.S. Const. art. 1, § 2.
The first Census was taken in 1790, and since that time the Census of population has been taken each year ending in zero.
Census Day is April 1, 2020, and the Census is designed to determine the population of the United States as of that date.
Census results are used to determine the number of congressional seats apportioned to each state.