On September 17, 1787, the Constitutional Convention completed its work and 39 delegates signed the Constitution of the United States. Constitution Day celebrates this auspicious event in the founding of the United States and the resilience and flexibility of the framers' work that we still interact with today.
"The Constitution of the United States of America is the supreme law of the United States. Empowered with the sovereign authority of the people by the framers and the consent of the legislatures of the states, it is the source of all government powers, and also provides important limitations on the government that protect the fundamental rights of United States citizens." - From WhiteHouse.gov