This is the day in 1787 that the Constitution was signed and ratified, formalizing the nation's government and decreeing that all who are born within the United States shall be citizens.
If you haven't read it lately (or at all), here is the first paragraph you still should have heard:
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.
Your history and civics lesson for the day. Boom.