Based on the united states constitution, the census to determine the number of
(1)presidential candidates
(2)members of congressional subcommittees
(3)electoral votes assigned to each state
(4)senators from each state

Based on the United States Constitution, the census helps to determine the number of (3) electoral votes assigned to each state.
The census determines the number of seats in the House of Representatives, which is limited to 435 in total. 

1920 census

the census was scheduled for january the 1st, as the government believed that more farmers would be in their homes by that time instead of april. the census did not ask about unemployment, the number of children, and how long was a couple married were not taken into consideration. individuals were considered as residents of the places they normally slept, not the places were they worked at or visited.

2010 census

the census was scheduled for april 1st, 2010. it was the shortest census in history: it only contained 10 questions related to name, age, gender, ethnicity, relationship, and whether the citizen owned a home or rented one. the government launched a website in order to provide the citizens with information regarding the census. there were also "teach census weeks" in schools nationwide.