Tomorrow the 1940 United States Population Census will be released. In order to find someone in this Census it is necessary to know the Enumeration district the person or family lived in at the time of the Census. My ancestors and immediate relatives are in three main locations: Hamilton County, Ohio and Whitley and McCreary Counties, Kentucky.
The Public Library of Cincinnati and Hamilton County has placed many Cincinnati City Directories online and these allow you to get the address where your relative was living. Once you have the address you can go to either the National Archives or to the 1940 Unified Finder Steven Morse One Step page to get the enumeration District number. With that number you can view the 1940 Census page to search that ED for your relatives.
Name indices are not expected to be available until later in April and not to be available for all of the USA and its Possessions until the end of the year.