On September 17, 1787, delegates to the Constitutional Convention in Philadelphia signed the final draft of the US Constitution. Since 1952, this date has been celebrated as Constitution Day. By law, all public schools that receive federal funding use this day to educate students about the Constitution.
The US Constitution documented a framework for how the new nation was to be governed. Since a government ruled by its people instead of a king was a completely unique idea at the time, there were a lot of things to work out. The original document contains seven articles and is rather short, so you can read it in one sitting. Article V sets out how the Constitution could be amended to update the document or address issues not already covered. The first ten amendments, known as the Bill of Rights, were ratified in 1791 to clarify specifics of the people's freedom from government interference. Since then, another 17 amendments have been added to address issues such as slavery, voting rights, alcohol prohibition and repeal, and taxes.
Schools, local governments, foundations, and other organizations are holding events across the country today to both teach and celebrate the US Constitution. You can attend one near you, or read about how the Constitution came about at Wikipedia in an article that is much, much longer than the Constitution itself.
(Image credit: Howard Chandler Christy)