Mardi Gras (French for "Fat Tuesday") happens every year on the Tuesday before Ash Wednesday, the date that begins the fasting season of Lent in the Western Christian tradition. But anyone who's been to Mardi Gras in New Orleans knows it's much more than that.
Basically, it's a celebration of the city itself. After all, the event dates back to 1699 — 19 years before New Orleans was even established. In fact, the city shuts down offices and schools to participate in the several parades, balls and other festivities that occur the weeks leading up to the big day.