Smithsonian Channel will mark the fifteenth anniversary of the 9/11 terrorist attacks with a special day of programming on Sunday, September 11. The marathon will kick off at 2:00 p.m. with an encore presentation of the critically-acclaimed 9/11: DAY THAT CHANGED THE WORLD, which spans from dawn to midnight on September 11, 2001, and goes behind the scenes to show how the events of the day developed.