Maybe it's partly a national thing. For Germany, one contender surely is 9 November:

1918 German capitulation ends WWI (a good thing, on the whole...)

1923 Hitler and associates stage the March on the Feldherrnhalle, Munich (and fail - a good thing in the short term only)

1939 State-engineered pogrom against the Jews known as 'Kristallnacht' in Germany (a thoroughly bad thing)

1989 The Berlin Wall comes down (the jury is still out on whether this was a bad or a good thing)

I'm sure Wolfgang or someone else will come up with more events.