The census is conducted in years ending in '0', and as such there is no census from 1956. Information on the census can be found at the United States Census Bureau website.
A statistical abstract of the United States for 1956 was produced by the Census Bureau, according to directory site The E EBook. This publication is based on data from the 1950 census and other records.
The decennial censuses for 1950 and 1960 can be found on the Census Bureau website. Census records can also be found on The Census, a website for finding family data and other records from the census.