Saturday marks twenty years since the 9/11 terror attacks. ABC's Aaron Katersky takes us back to that day with a riveting account of how that day unfolded, told through the stories of first responders, survivors and others. The last survivor, pulled from the rubble of the World Trade Center 27 hours after the attack, provides her first-hand account of that day. Finally, we revisit the young women and their then-infants left behind when their husbands and fathers were killed that day and how these widows have helped their children remember the fathers they've never met.