Glenn Fleishman writing in Smithsonian Magazine:

At midnight on New Year’s Eve, all works first published in the United States in 1923 will enter the public domain. It has been 21 years since the last mass expiration of copyright in the U.S.

We can blame Mickey Mouse for the long wait. In 1998, Disney was one of the loudest in a choir of corporate voices advocating for longer copyright protections. At the time, all works published before January 1, 1978, were entitled to copyright protection for 75 years; all author’s works published on or after that date were under copyright for the lifetime of the creator, plus 50 years. Steamboat Willie, featuring Mickey Mouse’s first appearance on screen, in 1928, was set to enter the public domain in 2004. At the urging of Disney and others, Congress passed the Sonny Bono Copyright Term Extension Act, named for the late singer, songwriter and California representative, adding 20 years to the copyright term. Mickey would be protected until 2024—and no copyrighted work would enter the public domain again until 2019, creating a bizarre 20-year hiatus between the release of works from 1922 and those from 1923.