On what date did the United States Constitution become effective?

The United States Constitution became effective on March 4, 1789.

That date marked the start of the new federal government under the Constitution, which replaced the Articles of Confederation. The Confederation Congress selected March 4 as the day for the new government to begin operating after the Constitution had secured the required ratifications.

The Constitution itself was signed on September 17, 1787, and New Hampshire became the ninth state to ratify it on June 21, 1788. That ninth ratification satisfied Article VII's threshold, but the government did not begin functioning under the document until the appointed date in 1789. The First Congress then convened in New York City.

March 4, 1789, should not be confused with April 30, 1789, when George Washington took the presidential oath of office. It also differs from September 17, now observed as Constitution Day, which commemorates the signing of the document in 1787 rather than the start of its operation.