November 22, 1963 is one of them, as President Kennedy was assassinated that Friday in Dallas, Texas. I was 11.
September 11, 2001 is another, as first one plane hits one of the World Trade Centre twin towers, and then a second hit the other. A third crashed into the Pentagon, and a fourth was taken down in Pennsylvania. The world changed that day.
Another is July 20, 1969. I was on vacation near Kennebunkport, Maine with my family, staying in a motel, and staying up late that night. The Lunar Module, nicknamed "The Eagle" landed on the surface of the moon. And hours later, astronauts Neil Armstrong and Buzz Aldrin climbed down a ladder while the world watched, and for the first time in history, human beings were walking on the surface of the moon. What an accomplishment.