Ancestry.com is providing free access to 1 billion hand-selected records from the 1940s. Anyone can use these records to find names, dates, births, marriages, military service and more about their families. Family history researchers can then use those details to locate their family in the 1940 Census. Access to the 1940's era records is free from April 2 through April 10.
Access to 1940s records is not the 1940 census, and it's only for a limited time (and if you're not a subscriber, you can't see details - you need to sign up for at least a trial subscription for that feature). Access is free for the 1940 census is only until the end of 1913. Researchers are constantly looking at old censuses(1930 and before), and for those they need a subscription.