On October 23, 1962, the US Post Office Department unknowingly issued an unknown number of inverted Dag Hammarskjöld error stamps. The fallout from this issue became known as Day’s Folly (after Postmaster General J. Edward Day). Dag Hammarskjöld was the second secretary-general of the United Nations, serving from April 1953 until his death in a plane crash in September 1961. President John F. Kennedy called him “the greatest statesman of our century.” To honor Hammarskjöld’s life and works, the US Post Office decided to issue a stamp in his honor the year after his death.