January 1, 1928. A day that will live in... famy? After fears that the public domain (PD) would remain frozen forever, it started... very slowly... moving forward again in 2019 in the United States. This year's Public Domain Day is unusually momentous, as a certain rodent you may know has finally been set loose. One of the driving forces behind multiple rounds of copyright extension in the United States over the last century was lobbying from the Disney corporation. The most recent of them, the 1998 Mickey Mouse Protection Act, was called that for a reason: repeated copyright extensions allowed Disney to keep their grasp on the oldest iterations of Mickey, whose first appearance was in the 1928 cartoon Steamboat Willie. It locked up tons of other harmless photographs and books in limbo as well, which was just too bad as far as Disney was concerned. However, we did not see another extension, and now Steamboat Willie has entered the public domain, along with a massive pile of other works.
More specifically: works published in 1928 are now unconditionally in the public domain in the United States (many lesser works were probably already public domain due to not being renewed, but now there's no need to verify the lack of renewal). In countries that use author's life plus some number of years, that means a few things: in "life + 70" countries, authors who died in 1953 had their works hit the public domain; and in "life + 50" countries, authors who died in 1973 have their works in the public domain. There are some complications here for Wikipedia though, which we will get into shortly.
This topic has been covered in a number of places, so here are some useful links: