Although the Constitution has been around for more than 200 years, Constitution Day is a relatively new phenomenon.
As the Senate explains: "To encourage all Americans to learn more about the Constitution, Congress in 1956 established Constitution Week, to begin each year on September 17th, the date in 1787 when delegates to the Convention signed the Constitution. In 2004, Senator Robert C. Byrd of West Virginia included key provisions in the Consolidated Appropriations Act of Fiscal Year 2005 designating September 17th of each year as Constitution Day and requiring public schools and governmental offices to provide educational programs to promote a better understanding of the Constitution."
If you'd like to explore this legislative history in more detail, start with the historical and revision notes for 36 U.S.C. § 106.
Not historical enough for you? The ABA notes that we are nearing the 800th anniversary of the Magna Carta, contributing to this year's designated theme, The Bill of Rights at 225.