Constitution Day, AKA Citizenship Day, is a federally-mandated American holiday, although it is not observed by the majority of the population. It is officially celebrated on September 17, which is of course, the day that the U.S. Constitutional Convention signed the Constitution in 1787. Because it is not actually 'celebrated' as such, if it falls on a weekend or on a better holiday schools will observe it on the closest school day. It is commonly used as an excuse to teach patriotism and social studies to bored students. This is not just a matter of persnickety principals grasping at straws; the law that established Constitution Day mandates that all federally-funded schools provide 'educational programming' on the American Constitution on this day.
Officially, this is the day that we celebrate the creation and signing of the Constitution of the United States of America, and therefore, we are also celebrating the American Citizen and all those people who have joined this group, whether through coming of age or by naturalization. Unofficially, we really celebrate all of this on the Fourth of July.
In practice, we celebrate Constitution Day by ignoring the educational programs and materials developed for this event by the Center for Civic Education, U.S. Department of Education, and local education and social organizations. As of yet, there are no American traditions or celebrations surrounding Constitution Day, although fireworks, cookouts, banners, and flags are all appropriate if you feel so inclined.
No one, not even federal workers, gets time off for this holiday.