Every generation has a tragedy where everyone seems to remember where they were when the heard. The most recent one was 9/11. My theory is that it happens once every 20 years or so. Here are the last four:
2001: September 11th attacks
1986: Challenger explosion
1963: JFK Assassination
1941: Pearl Harbor attack
Beyond that I'd only be speculating. Perhaps Black Monday in 1929? Or something WWI related (1914-1918)?
I can tell you the details of my day on 9/11/2001. That's the one I'm old enough to remember. I guess what I'm also trying to say is that we're 5-10 years away from another one. That's it. That's my theory. The every 2 decade tragedy theory.