Sept. 17, 1787: The Constitution of the United States is signed by 39 men after a summer of debate. It is then sent to the states for ratification.
“We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.”
Some cool Constitution-related websites:
National Constitution Center
Library of Congress
From the "coincidence or not?" department:
Sept. 17, 1796: President George Washington delivers his "Farewell Address" to the press before concluding his second term in office.