Technically, the magic date is actually 1976. That's when the term of US copyright first changed to the life + x formula used by the Berne Convention. However, it's not that simple.
Prior to 1976, under the 1909 US Copyright Act, the term of copyright was 20 years, renewable once and once only for another 20 years.
Of course that's complicated, by the 1998 Sonny Bono Copyright extension.
The 1976 copyright act brought US copyright law into closer(but not exact) alignment with the Bern Convention.
It increased the terms of copyright to author's life + 70 years and eliminated mandatory registration (under the 1909 act you had to register to have a copyright at all).
However, none of the changes in the 1976 act were made retroactive to works under copyright before 1976, and the 1976 act allowed all pre-1976 works still under their original 20 year term to be renewed per the 1909 act terms whenever they expired.
Then comes the 1998 Sonny Bono Copyright Act which extended the term of copyright to life + 100 years, and it was explicitly retroactive to all works still under active copyright.
So for any US works to be for certain off copyright they would have to be older than 1998-40 years or 1958 or earlier. however anything as late as 1976 could be public domain if the author failed to renew.
Renewal under the 1909 act was not automatic, you had to explicitly file for it and there were fees that had to be paid, both for the original registration and the renewal. So it's possible that some authors/publishers with 1959-1976 works that were not in active circulation post 1976 didn't bother to renew.