- The Oklahoma Land Rush began. (1889) Hundreds of thousands of settlers rushed into Oklahoma as soon as it officially opened on this day; within hours, cities of more than 10,000 people had formed.
- The first Earth Day was held. (1970) The day was the invention of Wisconsin Senator Gaylord Nelson, who saw it as a grassroots way of keeping the environment at the front of public consciousness. Millions participated in the first Earth Day, and by the 1990s, over 200 million people were participating annually in over 140 countries.
- The McCarthy Army hearings began. (1954) McCarthy had accused the Army of being "soft" on communism in an attempt to boost his waning popularity. It came out in the hearings that most of McCarthy's claims were unsubstantiated and, especially when combined with his own belligerent manner, McCarthy essentially lost all credibility.
- Seduction by teachers was outlawed in Ohio. (1886) The law prohibited even consensual sex between male teachers and adult female students. Ohio wasn't the first state to have seduction laws; several men were convicted in other states, and women sometimes used the laws to force men to marry them.
- German forces began using poison gas in combat. (1915) German forces began firing chlorine at Allied forces in the battle of Ypres, beginning one of the most devastating aspects of World War I. Chemical and biological weapons were used by both sides in the war, which led to them being mostly outlawed in warfare after World War I.
- Oil rig Deepwater Horizon sank, beginning the BP oil spill. (2010) The oil spill was considered one of the worst in environmental history, and devastated much of the Gulf of Mexico. BP came under intense criticism for its handling of the situation, and clean-up efforts lasted more than a year.
- Hitler admitted defeat. (1945) After living in the Fuhrerbunker for several months, Hitler admitted to his generals that defeat was inevitable and that suicide was his only recourse. He committed suicide eight days later, though the Fuhrerbunker remained in use for a little while afterwards.
- The United States Holocaust Memorial Museum in Washington D.C. was dedicated. (1993) The museum had been in the works since the 1970s, but was finally dedicated in 1993 by Bill Clinton, along with Chaim Herzog and Elie Wiesel. The museum's first visitor was the Dalai Lama.
- US Congress approved the use of "In God We Trust" on US coins. (1864) The Coinage Act of 1864 made it mandatory for all US coins to have the motto "In God We Trust," rather than "E Pluribus Unum" — "Out of Many, One".
- The Blues Brothers made their world premier. (1978) John Belushi and Dan Aykroyd made their world premier on Saturday Night Live as a pseudo-real blues band. They eventually became popular as a real commercial product, and made several albums along with a movie.