September 17 is designated as Constitution Day to commemorate the signing of the Constitution in Philadelphia on September 17, 1787. This webpage provides more information.
"In Philadelphia on May 14, 1787, the Federal Convention convened to revise the Articles of Confederation. But through discussion and debate it became clear that rather than amend the existing Articles, the Convention would draft an entirely new frame of government. All through the summer, in closed sessions, the delegates debated, and redrafted the articles of the new Constitution. Among the chief points at issue were how much power to allow the central government, how many representatives in Congress to allow each state, and how these representatives should be elected.
On September 17, 1787, the delegates to the Constitutional Convention met for the last time to sign the document they had created. September 17th is designated as Constitution Day to commemorate the signing."
From: DocsTeach: A Special Page Devoted to Teaching the U.S. Constitution
