When was the United States constitution ratified?
The Constitution of the United States was ratified on September
Actually, while the Constitution reached its final form on
September 17, 1787, it was, according to Article 7, not "ratified"
until at least 9 states had ratified it. The 9th state to do so was
New Hampshire, which ratified it on June 21, 1788.
The final state to ratify it was Rhode Island on May 29, 1790,
so you could also consider that a valid answer to the question.