John Kennedy actually said...
As a mark of respect to the memory of those who gave their lives when the U.S.S. THRESHER was lost off the Atlantic coast on Wednesday, April 10, 1963 it is hereby ordered pursuant to the provisions of Section 4 of Proclamation 3044 of March 1, 1954 that until sunset on Monday, April 15, 1963 the flag of the United States shall be flown at half-staff on all buildings, grounds and naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions.
04/11/1963