What date was the United States Constitution signed, a milestone included in America250 history?

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

Delegates signed the document at the Pennsylvania State House, now called Independence Hall, in Philadelphia. Thirty-nine delegates endorsed the completed Constitution at the end of the Constitutional Convention, which had met during the summer of 1787.

Signing did not immediately put the Constitution into effect. The document required ratification by conventions in at least nine states, and that process began with Delaware in December 1787. The new federal government began operating in 1789.

A common mix-up is treating September 17 as the Constitution’s ratification date. It is the signing date. Constitution Day in the United States is observed on September 17 because it commemorates the signing, while the Constitution became effective after the required state ratifications.