Question: Raymond Palmer co-wrote The Coming of the Saucers with an American aviator and businessman who made what is considered the first widely reported unidentified flying object sighting when?

Context paragraphs:
Cash-Landrum incident: The Cash-Landrum Incident was a highly reported unidentified flying object sighting from the United States in 1980, which witnesses insist was responsible for damage to their health.  It is one of very few UFO cases to result in civil court proceedings.

Raymond A. Palmer: Raymond Arthur Palmer (August 1, 1910 – August 15, 1977) was an American editor of "Amazing Stories" from 1938 through 1949, when he left publisher Ziff-Davis to publish and edit "Fate Magazine", and eventually many other magazines and books through his own publishing houses, including Amherst Press and Palmer Publications.  In addition to magazines such as "Mystic," "Search," and 'Flying Saucers," he published numerous spirtualist books, including "", as well as several books related to flying saucers, including "The Coming of the Saucers," co-written by Palmer with Kenneth Arnold.  Palmer was also a prolific author of science fiction and fantasy stories, many of which were published under pseudonyms.

Sperry UFO case: The Sperry UFO case was a sighting of an Unidentified Flying Object by the captain, Willis Sperry, and other crew of an American Airlines DC-6 airborne near Mount Vernon, Maryland on 29 May 1950.

World UFO Day: World UFO Day is an awareness day for people to gather together and watch the skies for unidentified flying objects.  The day is celebrated by some on June 24, and others on July 2.  June 24 is the date that aviator Kenneth Arnold reported what is generally considered to be the first widely reported unidentified flying object sighting in the United States, while July 2 commemorates the supposed UFO crash in the 1947 Roswell UFO Incident.

Flying saucer: A flying saucer (also referred to as a flying disc) is a descriptive term for a supposed type of flying craft having a disc or saucer-shaped body, commonly used generically to refer to an anomalous flying object.  The term was coined in 1930 but has generally been supplanted since 1952 by the United States Air Force term unidentified flying objects or UFO's.  Early reported sightings of unknown "flying saucers" usually described them as silver or metallic, sometimes reported as covered with navigation lights or surrounded with a glowing light, hovering or moving rapidly, either alone or in tight formations with other similar craft, and exhibiting high maneuverability.

1976 Tehran UFO incident: The 1976 Tehran UFO Incident was a radar and visual sighting of an unidentified flying object (UFO) over Tehran, the capital of Iran, during the early morning hours of 19 September 1976.  During the incident, two F-4 Phantom II jet interceptors reported losing instrumentation and communications as they approached the object.  These were restored upon withdrawal.

Nash-Fortenberry UFO sighting: The Nash-Fortenberry UFO sighting was an unidentified flying object sighting that occurred on July 14, 1952, when two commercial pilots (William B. Nash and William H. Fortenberry) claimed to have seen eight UFOs flying in a tight echelon formation over Chesapeake Bay in the state of Virginia.

Kenneth Arnold: Kenneth A. Arnold (March 29, 1915 – January 16, 1984) was an American aviator and businessman.  He is best known for making what is generally considered the first widely reported unidentified flying object sighting in the United States, after claiming to have seen nine unusual objects flying in tandem near Mount Rainier, Washington on June 24, 1947.

Mariana UFO incident: The Mariana UFO Incident occurred in August 1950 in Great Falls, Montana.  The film footage of the sighting is believed to be among the first ever taken of what came to be called an unidentified flying object, and was investigated by the US Air Force.

Edward J. Ruppelt: Edward J. Ruppelt (July 17, 1923 – September 15, 1960) was a United States Air Force officer probably best known for his involvement in Project Blue Book, a formal governmental study of unidentified flying objects.  He is generally credited with coining the term "unidentified flying object", to replace the terms "flying saucer" and "flying disk" - which had become widely known - because the military thought them to be "misleading when applied to objects of every conceivable shape and performance.  For this reason the military prefers the more general, if less colorful, name: unidentified flying objects.  UFO (pronounced Yoo-foe) for short."

Supporting paragraphs: Raymond A. Palmer Kenneth Arnold
Answer: June 24, 1947