Signing of the United States Constitution
On September 17, 1787, at Independence Hall in Philadelphia, Pennsylvania, 39 delegates to the Constitutional Convention endorsed the Constitution created during the four-month-long convention. The U.S. Constitution lays out the frame of the nation's federal government and delineates how its three branches (legislative, executive, and judicial) are to function.
Source: Wikipedia — Signing of the United States Constitution (CC BY-SA 4.0)