The last Census in the United States was conducted in the year 2010 (population data from 2010). The next Census will be in the year 2020. The Census is taken every 10 years; this schedule is required by the US constitution:
The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.
The constitution was written in 1787, thus the first Census was in 1790. There has been a Census every 10 years since 1790 (1800, 1810, etc).
The main purpose of the Census is also described by the US constitution:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers
Because the United States is a representative democracy, the Census ensures that every person is fairly represented in Congress. So the point of the Census is to regularly count the population of every state, so a fair numbers of representatives can be apportioned (allocated) to each state. The count of representatives for each state is incredibly important because the number of votes in the electoral college is tied directly to the count of representatives in Congress.