The United States Constitution’s government replaced the Articles of Confederation on March 4, 1789.
The Constitution had been signed in Philadelphia on September 17, 1787, and reached the required nine-state ratification threshold on June 21, 1788. Those milestones did not immediately replace the existing government. The Constitution specified that the new federal system would begin operating when the first Congress assembled, which occurred on March 4, 1789.
The transition created three branches: Congress to legislate, an executive headed by a president, and a federal judiciary. George Washington was inaugurated as the first president on April 30, 1789, in New York City, the temporary national capital.
This timeline explains why several dates appear in accounts of the Constitution. Signing created the document, ratification approved it, and March 4 marked the start of its government. The Articles therefore remained the operating framework between 1781 and early 1789.