Answer:
JFK's assassination
Step-by-step explanation:
The president of the United States, John F. Kennedy (JFK) was assassinated while he was visiting Dallas, Texas, on November 22, 1963.
It's still considered an unsolved murder by many people, since the alleged killer, Lee Harvey Oswald, was killed a few hours after being arrested, before his trial.
The assassination marked the collective memory because it was essentially presented live on TV. Everyone alive then remember where they were when they heard the news, because it shocked the whole world, not just the United States.
He's the last of 4 presidents assassinated while in office. The first one was one was Abraham Lincoln (1865), then James A. Garfield (1881) and William McKinley (1901).