September 17, 1787. The United States Constitution was adopted by the Constitutional Convention (or Philadelphia Convention) in Philadelphia. It then went on to be ratified by each of the 13 states in the name of "The People".
The Convention took place from May that year until the 17th of September. George Washington was elected president of the convention. The debates were kept secret from the public, so that the delegates could speak freely. Every state sent delegates, except for Rhode Island.
The purpose of the convention was to replace the Articles of Federation, which was ratified seven years earlier and was the first constitution of the United States of America.
The Constitution would not take effect until nine states had ratified it. On June 21, 1788 New Hampshire became the ninth state to ratify the Constitution. Once the Congress of the Confederation received confirmation, they started planning for the start of operations under the Constitution to begin. On March 4, 1789 the Constitution went into effect and the Congress of the Confederation was hereby known as the United States Congress.
Constitution of the United States - Official
United States Constitution: Primary Documents of American History