Mardi Gras, aka "Fat Tuesday," is celebrated today. Tomorrow is Ash Wednesday, the first day of the fasting season of Lent. Big time celebrations will occur in New Orleans, Rio de Janeiro, Sao Paulo, Mobile, Galveston, and elsewhere ... it's traditional.
We actually experienced the final day of Carnival as it's known in Rio, and a bigger party would be hard to imagine. Imagine a million happy drunks, bouncing in the humid summer dark to a samba beat, as they watch the winning samba clubs or "schools" strut their completely over-the-top stuff. Check out the Rio photos and video online, at Youtube, for instance.
In New Orleans the similar clubs are called "krewes" and the beat is more Dixieland than samba. In this hemisphere we do Lent in winter, south of the equator, it happens in summer - same date, opposite season.