On May 20, ...

... 1803, in what then was Lower Canada and now is Québec, Chief Justice William Osgoode ruled that "slavery was not compatible with British law," thus limiting slave trafficking in the province.
...1902 (105 years ago today), the Republic of Cuba was established, bringing an end to U.S. occupation that had persisted since the end of the Spanish-American War in 1898.
... 1989, in the face of pro-democracy demonstrations in Beijing's Tiananmen Square (right), Chinese leaders declared martial law. Protests would continue throughout the month.