As of 2019, copyright has expired for all works published in the United States before 1924. In other words, if the work was published in the U.S. before January 1, 1924, you are free to use it in the U.S. without permission. These rules and dates apply regardless of whether the work was created by an individual author, a group of authors, or an employee (a work made for hire).
Because of legislation passed in 1998, no new works fell into the public domain between 1998 and 2018 due to expiration. In 2019, works published in 1923 expired. In 2020, works published in 1924 will expire, and so on.
For works published after 1977, if the work was written by a single author, the copyright will not expire until 70 years after the author’s death. If a work was written by several authors and published after 1977, it will not expire until 70 years after the last surviving author dies.
Copyright protection always expires at the end of the calendar year of the year it’s set to expire. In other words, the last day of copyright protection for any work is December 31. For example, if an author of a work died on June 1, 2000, protection of the works would continue through December 31, 2070.
Thousands of works published in the United States before 1964 fell into the public domain because the copyright was not renewed in time under the law in effect then. If a work was first published before 1964, the owner had to file a renewal with the Copyright Office during the 28th year after publication. No renewal meant a loss of copyright. If you plan on using a work that was published before 1964, you should research the records of the Copyright Office to determine if a renewal was filed.