Constitution Day is September 17, 2015. Celebrate the ratification of the U.S. Constitution. The Constitution of the United States of America is the supreme law of the United States. It is the foundation and source of the legal authority underlying the existence of the United States of America and the Federal Government of the United States. The United States Constitution was adopted on September 17, 1787, by the Constitutional Convention in Philadelphia, Pennsylvania. Learn about the U.S. Constitution, the Bill of Rights, Amendments to the Constitution, and our Founding Fathers. Extensive collection of constitutional books and DVDs.