A history of the holiday. On September 17, 1787, the delegates to the Constitutional Convention met for the last time in Philadelphia to sign the document they had created. However, the first observance of the anniversary of the signing of the Constitution of the United States was held over 70 years later, shortly after the outbreak of the Civil War. Then in 1940, Congress set aside the third Sunday in May as “I Am an American” Day, which honored those who had become U.S. citizens during the preceding year. These two holidays were combined in 1952 and were celebrated in that year on September 17th as Citizenship Day. The holiday is presently referred to by both Consitution Day and Citizenship Day.
Interestingly, although Convention delegates put four months of negotiating into the document, ratification by the last of the states took two and a half years to achieve and the Constitution that was finally ratified by the 13th state (Rhode Island) on May 29, 1790, had been substantially amended by that time.