September 16 was Independence Day in Mexico, while up north, today is Constitution Day, which used to be Citizenship Day, and is actually being celebrated all week.
In 1787 on this day, the 55 delegates of the Constitutional Congress finally agreed on that oft-misquoted document, our Constitution, and submitted it to the states for ratification.
The Nashua Telegraph sorts it out:
Constitution Day became a holiday in 2004 with the passage of an amendment by Sen. Robert Byrd, D-W.Va. He inserted the amendment because of the apparent ignorance many Americans display in regard to their nation’s history. Prior to being known as Constitution Day, Sept. 17 was referred to as Citizenship Day. Beyond changing the name of the holiday, the amendment required all schools receiving federal funds to offer educational programming about the Constitution every Sept. 17.
Our current executive branch manager declared the holiday anew, as presidents, governors and mayors are are wont to do, despite recent minterpretation of its delegation of powers:
America is grateful to those who have worked to defend the Constitution and promote its ideals. During this observance, we also recognize the profound impact our Constitution has on the everyday lives of our citizens, and we call upon all Americans to help uphold its values of a free and just society.
If you can't name the rights in the First Amendment or the Amendments in the Bill of Rights or need help with that Preamble you memorized in grade school, Cornell Law School's site is one place to study.
There are a number of other annotated sites out there, and the official Constitution Center's site has downloads, an image of the four-parchment original, and even translations in Spanish, Arabic, French, German, Italian, Japanese, Korean, Portuguese, Russian and Simplified Chinese.