On what date did delegates sign the U.S. Constitution?

Delegates signed the U.S. Constitution on September 17, 1787.

The signing took place at the Pennsylvania State House in Philadelphia, now known as Independence Hall, at the close of the Constitutional Convention. George Washington presided over the convention, and 39 of the 42 delegates still present signed the document. Three delegates—George Mason, Edmund Randolph, and Elbridge Gerry—refused to sign.

The signing completed the convention’s work but did not immediately create an operating federal government. The Constitution still required ratification by nine states. It reached that threshold in June 1788, and the new government began operating in March 1789.

September 17 is now observed in the United States as Constitution Day and Citizenship Day. The date is sometimes confused with July 4, 1776, which marks adoption of the Declaration of Independence, or with June 21, 1788, when the ninth state ratified the Constitution.