On September 17, 1787 the members of the U.S. Constitutional Convention signed the Constitution into law. Every year on September 17th we thus celebrate Constitution Day and Citizenship Day, and in fact the date kicks off Constitution week. Each year there is a presidential statement reaffirming the holiday and educational institutions are encouraged to offer lessons on constitutional history. We sometimes forget that all of the law that we study and practice has its roots in the United States Constitution.
If you’d like to brush up on the Constitution, you can find a copy here. The National Archives also has some interesting materials on U.S. Constitutional history.