On what date was the U.S. Constitution signed at the Philadelphia convention?

The U.S. Constitution was signed at the Philadelphia convention on September 17, 1787.

The signing took place at the Pennsylvania State House, now known as Independence Hall, during the final session of the Constitutional Convention. Thirty-nine delegates signed the document, representing 12 states; Rhode Island had declined to send delegates. George Washington, the convention's president, signed first, followed by the other delegates.

Signing did not immediately make the Constitution the operating law of the United States. Article Seven sent the document to the states for ratification, and nine states had to approve it before it could take effect. New Hampshire supplied the decisive ninth ratification on June 21, 1788. The Constitution then established the new federal government, which began operating in 1789.