The day the Constitution was ratified - National Constitution Center

On June 21, 1788, the United States Constitution went into effect when New Hampshire became the 9th state to ratify it. From the article:

"On June 21, 1788, the Constitution became the official framework of the government of the United States of America when New Hampshire became the ninth of 13 states to ratify it. The journey to ratification, however, was a long and arduous process.

Until the new Constitution was ratified, the country was governed by the Articles of Confederation. That document was tailored to a newly formed nation made of states acting more like independent, sovereign countries, and it quickly became clear to some of America’s leaders that future stability required a stronger, more centralized government. New York’s Alexander Hamilton thus led the call for a constitutional convention to reevaluate the nation’s governing document. The Confederation Congress endorsed his initiative, and representatives from all 13 states were subsequently invited to convene in Philadelphia on May 25, 1787, to participate in the Convention.

The initial purpose of the Convention was for the delegates to amend the Articles of Confederation; however, the ultimate outcome was the proposal and creation of a completely new form of government. Three months later, on September 17, 1787, the Convention concluded with the signing (by 38 out of 41 delegates present) of the new U.S. Constitution. Under Article VII, it was agreed that the document would not be binding until its ratification by nine of the 13 existing states."