In the beginning, everything was in the Public Domain. But that all changed when the English Queen Anne put an end to public ownership of our shared culture by passing the first Copyright Law in 1710. The idea was that this would encourage creators to create. Initially this Intellectual Property monopoly applied exclusively to the printed word. The term was limited to a few years to ensure creative works would return to the Public Domain.
As time went by, however, the scope of copyright has expanded to include most of the creative realms, and what were once limited terms (ostensibly intended to encourage creators to create) now extends decades past the death of the author. (So far no one has explained how this can possibly encourage dead creators to create new art.)
Because copyright terms have been extended so long, and sometimes even retroactively, works going into the Public Domain have flowed to a trickle, and in some cases to a halt. Those works that are emancipated from copyright bondage are scheduled to enter the Public Domain on January 1st every year, so on this day we celebrate the expiry of the monopoly over the works that return to the Public Domain.