On February 18, 1965, a young man named Jimmie Lee Jackson was shot and killed by a member of the Alabama State Police during a non-violent civil rights demonstration in Selma, Alabama.
Seventeen days later, 525 civil rights activists marched from Selma to Montgomery, Alabama, in protest of that killing. They were attacked by state and local police armed with billy clubs, whips, and tear gas. (You can read the New York Times' entire horrifying account here.) That day—March 7, 1965—would come to be known as "Bloody Sunday."
Here is President Obama's statement marking the 49th anniversary:
Forty-nine years ago, a determined group of Americans marched into history, facing down grave danger in the name of justice and equality—walking to protest the continued discrimination and violence against African Americans. On a day that became known as “Bloody Sunday”, these brave men and women met billy-clubs and tear gas with courage and resolution. Their actions helped set an example for a generation to stand up for the fundamental freedoms due to all people. We recognize those who marched that day—and the millions more who have done their part throughout our nation’s history to bend the arc of the moral universe toward justice.