What was the significance of the Marian Anderson concert at the National Mall?

In 1939 Marian Anderson performed on the steps of the Lincoln Memorial after being denied the right to perform at Constitution Hall because she was Black. The concert was attended by 75,000 people and was the first of many events on the Mall that marked milestones in the civil rights movement, including the 1963 March on Washington and the 1995 Million Man March.