Answer:
"Bloody Sunday" refers to the March 7, 1965, civil rights march that was supposed to go from Selma to the capitol in Montgomery to protest the shooting death of activist Jimmie Lee Jackson. The roughly 600 marchers were violently driven back by Alabama State Troopers, Dallas County Sheriff's deputies, and a horse-mounted posse after they crossed the Edmund Pettus Bridge. The state and county officers beat and gassed the unarmed marchers in an attack, and media coverage of the event shocked the nation and led ultimately to the passage of the Voting Rights Act of 1965. The descriptive term appeared in relation to the events within days in the national media.