Jack Dorsey sends the world's first Twitter message, or tweet. The microblogging service revolutionized the communication and social networking landscape. In 2012, about 340 million tweets were posted per day.

South African Police kill at least 21 black people commemorating a similar mass shooting 25 years before. The Sharpeville massacre in 1985 had left 69 unarmed people dead. It was a turning-point in the history of South Africa.

Earth Day is celebrated for the first time. The first edition was limited to some cities in the United States. Today, Earth Day is observed by about 1 billion people around the world.

The world's first rock and roll concert is held in Cleveland, Ohio. DJ Alan Freed presented the concert, which was closed down after only one song because of over-crowding.

A plot to assassinate Adolf Hitler by suicide bomb fails. German Wehrmacht officer, Rudolf von Gersdorff, failed to blow up the dictator but managed to defuse his bombs just before they went off and avoid suspicion.