The World UFO day is observed on the 2nd of July each year. It commemorates the supposed UFO crash at Roswell, New Mexico. The Roswell UFO incident, which happened in 1947, was the first widely reported event concerning UFOs.
There is an alternate UFO day which is celebrated on 24th June. It was on this day that US aviator, Kenneth Arnold, claimed to have seen nine objects in the sky flying in tandem near Mount Rainier, Washington in 1947.
