Where were you when?

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.