Interesting to see most people's choices are from the Twentieth Century. Mine are going to follow suit and are all fairly common suggestions I'd have thought:
-1966 World Cup Final
-1963 JFK assassination
-1969 Man landing on the moon
-1969 The Beatles play on the Apple rooftop
-1918 Armistice Day
I did see the beatles on the roof, I was working in Conduit street and we,saw and heard it all, we had agreat view from the roof of the building we were in ,lucky me, are you jealous ???