Here's my categorization:
End of WW1
1920s
Black Friday
1930s
Pearl Harbor
1940s
Beginning of Korean War
1950s
Deaths of Buddy Holly, Ritchie Valens, and The Big Bopper
1960s
Woodstock
1970s
Disco Demolition Night
early 1980s
Revolution Summer
late 1980s
Fall of Soviet Union
1990s
9/11
2000s
Death of Osama bin Laden
2010s
Murder of George Floyd
2020s
I am kind of sympathetic though to an argument I've heard that there's a period roughly coterminous with the George HW Bush presidency that is neither truly 80s nor truly 90s and should be considered its own thing.
If we're talking about music-adjacent stuff, I think the Manson Family murders should be the end of the 60s, not Woodstock.
The peace & love stuff at Woodstock doesn't really set the stage for the 70s very well, even though it may serve as a good ending to the 60s.