1947.06.12
A chain of flying objects was seen from Weiser, Idaho on 12 June.
(UFOs 1947-1987: The 40-Year Search for an Explanation, Evans and Spencer)
A… case, dated June 12, 1947, came from Weiser, Idaho (with Weiser being redacted from the Blue Book master index for some reason). According to the information, Mrs H Erickson (identified that way as a convention of the time) said that she had seen two high-speed, round objects at a high altitude. They were flying in a trail formation and left a vapor trail that persisted from more than an hour.