On what date was the United States Constitution signed, a foundational event commemorated by America250?

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

The signing took place at Independence Hall in Philadelphia during the final session of the Constitutional Convention. Thirty-nine delegates endorsed the document, representing twelve states; Rhode Island had declined to send delegates. George Washington, who presided over the convention, signed first.

Signing did not immediately mean that the Constitution governed the country. Article VII required ratification by conventions in at least nine states, and the new federal government began operating in 1789. The date is also distinct from June 21, 1788, when New Hampshire supplied the ninth ratification, and from March 4, 1789, when the Constitution’s government officially began operations.