The U.S. Constitution superseded the Articles of Confederation on March 4, 1789.
That date marked the beginning of operations under the new federal government. The Constitution had been drafted and signed in Philadelphia on September 17, 1787, but it did not immediately replace the Articles. Article Seven required ratification by nine states, a threshold reached when New Hampshire approved the document on June 21, 1788.
The Confederation Congress selected March 4, 1789, as the date for commencing proceedings under the Constitution. The first Congress eventually met in New York City, and George Washington took the presidential oath on April 30, 1789.
The date is sometimes confused with September 17, Constitution Day, or June 21, the ninth-state ratification date. Those events were important, but March 4 marks the launch of the new constitutional government.