The Dag Hammarskjöld invert is a 4 cent value postage stamp error issued on 23 October 1962 by the United States Postal Service (then known as the Post Office Department) one year after the death of Dag Hammarskjöld, Secretary-General of the United Nations, in an airplane crash. The stamp, showing the yellow background inverted relative to the image and text, is also known as the Day's Folly after Postmaster General J. Edward Day who ordered the intentional reprinting of the yellow invert commenting, "The Post Office Department is not running a jackpot operation."