On what date was the United States Constitution signed?

The United States Constitution was signed on September 17, 1787.

Delegates signed the document at the Pennsylvania State House in Philadelphia, the building now known as Independence Hall. The Constitutional Convention had begun on May 25, 1787, initially with the stated purpose of revising the Articles of Confederation. Instead, the delegates produced an entirely new framework for the federal government.

Thirty-nine of the convention’s 55 delegates signed the Constitution. Several delegates refused to sign, including George Mason, Edmund Randolph, and Elbridge Gerry, while other delegates had already left Philadelphia. The document then went to the states for ratification rather than taking effect immediately.

September 17 is observed in the United States as Constitution Day and Citizenship Day. The signing date is distinct from June 21, 1788, when New Hampshire became the ninth state to ratify, and March 4, 1789, when the new constitutional government began operating. Those dates are related but answer different historical questions.