On what date was the U.S. Constitution signed at the Constitutional Convention?

The U.S. Constitution was signed at the Constitutional Convention on September 17, 1787.

The signing took place at the Pennsylvania State House in Philadelphia, the building now known as Independence Hall. Delegates had met there during a four-month convention to replace the weak Articles of Confederation with a stronger federal framework.

Thirty-nine delegates signed the final document. They represented twelve states; Rhode Island declined to send delegates to the convention. Three delegates present at the convention refused to sign, including George Mason, Edmund Randolph, and Elbridge Gerry.

Signing did not make the Constitution immediately effective. Article VII required ratification by nine states. New Hampshire supplied the decisive ninth ratification on June 21, 1788, and the new federal government began operating under the Constitution on March 4, 1789. The signing date is now observed in the United States as Constitution Day.