Martin Luther King, Jr. gave what many consider as his most important speech on August 28th, 1963 at the Lincoln Memorial. The U.S. State Department's website ( http://usinfo.state.gov/usa... )describes the day:

"On August 28, 1963, under a nearly cloudless sky, more than 250,000 people, a fifth of them white, gathered near the Lincoln Memorial in Washington to rally for 'jobs and freedom.' The roster of speakers included speakers from nearly every segment of society -- labor leaders like Walter Reuther, clergy, film stars such as Sidney Poitier and Marlon Brando and folksingers such as Joan Baez. Each of the speakers was allotted fifteen minutes, but the day belonged to the young and charismatic leader of the Southern Christian Leadership Conference."

"Dr. Martin Luther King Jr. had originally prepared a short and somewhat formal recitation of the sufferings of African Americans attempting to realize their freedom in a society chained by discrimination. He was about to sit down when gospel singer Mahalia Jackson called out, 'Tell them about your dream, Martin! Tell them about the dream!' Encouraged by shouts from the audience, King drew upon some of his past talks, and the result became the landmark statement of civil rights in America -- a dream of all people, of all races and colors and backgrounds, sharing in an America marked by freedom and democracy."

The following video is a rarely seen version of Martin Luther King, Jr.'s "I have a Dream" speech: