Constitution Day! From HolidayInsights.com.
On February 29, 1952, President Harry Truman signed a bill establishing Citizenship Day on September 17 of each year. The roots of this holiday go back to I Am an American Day, which was established in 1940 by Congress as the third Sunday in May. This day was moved and renamed to Citizenship Day to coincide with the signing of the U.S. Constitution on September 17, 1787.