Like patent protection, copyright protection is limited in time. For example, a work published in the United States is eligible for copyright protection for the life of the author plus 70 years. Anonymous works, pseudonymous works, and works made for hire are eligible for a term of the shorter of 95 years from publication or 120 years from creation. For more information about copyright term, see here.
For example, a work authored by Author X in 1980, who died on May 4, 1994, would be eligible for copyright protection until May 4, 2064. And due to 17 USC § 305, the term of copyright would extend throughout the remainder of 2064. So, the work would enter the public domain on January 1, 2065.
However, copyright term has not always been so long. For example, a work published before the 1976 Copyright Act extended copyright term would have originally been eligible for 56 years of copyright protection (an initial 28-year term plus a 28-year renewal). Under this pre-1976 regime, works published in 1956 would have entered the public domain on January 1, 2013. Due to the 1976 Copyright Act these works will not enter the public domain until January 1, 2052 (95 years from publication, provided that certain formalities were met and the copyright was renewed after the initial 28 year term).
Center for the Study of the Public Domain
Every year, Duke University’s Center for the Study of the Public Domain releases a list of famous works that would have entered the public domain on January 1st under the pre-1976 Copyright Act regime.
Works that would have entered the public domain on January 1st, 2013 include famous books (e.g., Philip K. Dick’s “Minority Report”), famous periodicals (e.g., the first issue of MAD magazine), famous movies (e.g., The King and I starring Yul Brynner), and famous songs (e.g., In the Still of the Night performed by The Five Satins).
For the full list see the Center’s post at “What Could Have Entered the Public Domain on January 1, 2013?” Also, for more information about copyright and the public domain, see the “Public Domain Day Frequently Asked Questions.”