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

The story behind the answer

Construction of the Pentagon began on September 11, 1941, as the U.S. ramped up mobilization just before entering World War II. Built to consolidate the War Department’s scattered offices, it rose rapidly on the Virginia side of the Potomac. Its distinctive five-sided design came from early site constraints and traffic patterns, and it became (and remains) one of the world’s largest office buildings.

To remember the date, note the powerful coincidence: exactly 60 years later, on September 11, 2001, the Pentagon was attacked during the 9/11 terrorist attacks. Also, don’t confuse the start date with January 15, 1943—that’s when the Pentagon was officially dedicated, after its astonishingly fast wartime construction.