July 7 - 1947 - The Roswell Incident
On a summer day in 1947, some people found some debris outside of the town of Roswell, New Mexico. After said debris was given over to the US Air Force, the only thing the government would say was that some stuff was found and they knew what was going on. That largely ended the story for a time, but in the 1970s and 1980s this explanation sounds fishy to a specific subset of researchers. And that's how the most famous UFO story in American culture gets launched.