**ADVANCE FOR WEEK OF SUNDAY APRIL 6-SATURDAY APRIL 12.** Dr. Martin Luther King Jr., attends a news conference in Birmingham, Ala. in this May 9, 1963 file photo. Dr. Martin Luther King, Jr., was assassinated when he was only 39. Now, four decades later, the anniversary of his death is marked by a documentary that explores his life and legacy. "King," which airs Sunday at 8 p.m. EDT on History, revisits signal moments from his fight for civil rights: the bus boycott in Montgomery, Ala.; the march in Selma; his incarceration in Birmingham; the march on Washington and his speech where he declared unforgettably, "I have a dream." His tragic death in Memphis. (AP Photo/files)