"Representation and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers ... . 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."
-- Article I, Section 2, Constitution of the United States
The U.S. Census Bureau conducts the Decennial Census every 10 years as well as a number of other useful surveys such as the American Community Survey and the Economic Census.
The U.S. Census Bureau is a great source of demographic data, but it's not the only source. If the Census doesn't have the data you want, it may be collected by another government agency, poll or research project.