On what date was the United States Constitution signed, a key document in the America250 story?

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

Delegates at the Constitutional Convention in Philadelphia completed and signed the proposed Constitution on that date. Thirty-nine of the 42 delegates present signed it, while three—George Mason, Elbridge Gerry and Edmund Randolph—declined.

The Constitution did not become effective simply because delegates signed it. It required ratification by conventions in at least nine states. New Hampshire became the ninth state to ratify on June 21, 1788, allowing the new government to proceed.

The Constitution replaced the Articles of Confederation with a stronger federal system divided among legislative, executive and judicial branches. The new government began operating in 1789, with George Washington inaugurated as the first president.