On this day, July 20 ...

1969: Astronauts Neil Armstrong and Edwin "Buzz" Aldrin become the first men to walk on the moon after reaching the surface in their Apollo 11 lunar module.

Also on this day:

1923: Mexican revolutionary leader Pancho Villa is assassinated by gunmen in Parral.

1944: An attempt by a group of German officials to assassinate Adolf Hitler with a bomb fails as the explosion only wounds the Nazi leader.

An attempt by a group of German officials to assassinate Adolf Hitler with a bomb fails as the explosion only wounds the Nazi leader. 1968: The first International Special Olympics Summer Games, organized by Eunice Kennedy Shriver, are held at Soldier Field in Chicago.

The first International Special Olympics Summer Games, organized by Eunice Kennedy Shriver, are held at Soldier Field in Chicago. 1976: America's Viking 1 robot spacecraft makes a successful, first-ever landing on Mars.

America's Viking 1 robot spacecraft makes a successful, first-ever landing on Mars. 1977: A flash flood hits Johnstown, Penn., killing more than 80 people and causing $350 million worth of damage.

A flash flood hits Johnstown, Penn., killing more than 80 people and causing $350 million worth of damage. 1977: The U.N. Security Council votes to admit Vietnam to the world body.

The U.N. Security Council votes to admit Vietnam to the world body. 1982: Irish Republican Army bombs explode in London parks, killing eight British soldiers, along with seven horses belonging to the Queen's Household Cavalry.

1990: Supreme Court Justice William J. Brennan, one of the court's most liberal voices, announces he is stepping down.

1993: White House deputy counsel Vincent Foster Jr., 48, is found shot to death in a park near Washington, D.C.; his death is ruled a suicide.

2012: Gunman James Holmes opens fire inside a crowded movie theater in Aurora, Colo., during a midnight showing of "The Dark Knight Rises," killing 12 people and wounding 70 others. (Holmes would be convicted of murder and attempted murder, and sentenced to life in prison without the possibility of parole.)