9/11. I was driving to work in Kansas City, and turned on the radio. Horrified to hear the reports of hijackings, I heard the broadcasters as the first plane hit the North Tower. When I arrived at work and entered my workroom, my coworkers were standing around staring at the TV. I joined them in time to watch the second plane hit the South Tower. No one got any work done that day. We just watched the news - and also heard as the Pentagon was hit, and then the downed plane in the Pennsylvania field.
We'd heard that all planes, every airport, were grounded that day. NO ONE in the air. Except the President, of course. We heard they picked him up from that school in Florida, but no one knew exactly where he was headed. We thought it must be Camp David. I stepped outside the building for a smoke, and looked up to see a single airplane and contrail overhead, headed north. whaaaa??? We learned later they'd taken Air Force One to Texas for refueling, then were headed north to somewhere in Iowa(?) I think. Ferrying him from place to place, not knowing if the White House or Camp David would be next. So I watched him fly overhead, the only plane in the skies after the attacks.
I was privileged later to visit the Pentagon after the repairs had been made. They have a room and memorial dedicated to the people who lost their lives that day, and the first responders who tried their best.