Constitution Day commemorates the day that the Constitution of the United States was signed on September 17, 1787.
In 2004, an amendment to Title 36 Patriotic and National Observances, Ceremonies & Organizations, Section 106, changed the observance of Citizenship Day to Constitution Day and Citizenship Day. Part of the amendment requires that schools and educational institutions mark the day with education programs/activities remembering the privileges of citizenship in the United States of America (Library of Congress, 2019).
See the amendment to Title 36 below: