The Constitution of the United States of America
In 1940, following a push by the Hearst newspapers, Congress established the third Sunday in May as "I Am An American Day," a day for celebrating citizenship in the United States. In 1952, President Harry Truman moved the holiday to September 17th, and renamed it Citizenship Day. Its purpose was to "[commemorate] the formation and signing on September 17, 1787, of the Constitution and [recognize] all who, by coming of age or by naturalization, have become citizens."
Starting in 1956, Constitution Week was added, celebrating the Constitution from September 17 through September 23.
At the end of 2004, it was decided to redesignate September 17 as "Constitution Day and Citizenship Day," adding the additional observance of holding educational programs on the United States Constitution in all publicly-funded schools.