On which day did construction start on "The Pentagon", the headquarters for the United States Department of Defense?

Construction on the Pentagon, the headquarters for the United States Department of Defense, began on September 11, 1941. The reason for this date is that the groundbreaking ceremony for the building took place on that day. The Pentagon was constructed to provide a centralized location for the military administration during World War II and to accommodate the growing needs of the Department of Defense. Sadly, many years later, September 11 became a day of tragedy and remembrance due to the terrorist attacks in 2001 on the Twin Towers in New York City, the Pentagon, and United Airlines Flight 93.