On what date was the U.S. Constitution signed, a founding milestone highlighted by America250?

The U.S. Constitution was signed on September 17, 1787.

Delegates to the Constitutional Convention signed the document at the Pennsylvania State House in Philadelphia, now known as Independence Hall. The convention had met from May 25 to September 17, 1787, to revise the Articles of Confederation, but it ultimately produced an entirely new plan of government.

Thirty-nine delegates signed the Constitution on the final day. George Washington, who presided over the convention, signed first. Several delegates refused to sign, including George Mason, Edmund Randolph, and Elbridge Gerry, because they objected to parts of the proposed system or the absence of a bill of rights.

Signing did not immediately make the Constitution operative. It required ratification by conventions in nine states, achieved on June 21, 1788, when New Hampshire became the ninth. The new federal government began operating in 1789, while the Bill of Rights was ratified in 1791.