Mardi Gras is always the day before Ash Wednesday. It's the last day of the Csrnival season. This Tuesday is Fat, because this is the last day to eat, drink and be merry before the beginning of Lent and Ash Wednesday.
. Lundi Gras is a modern name invention. It means Fat Monday and is the day before Mardi Gras. It used to be a relatively slow day in the Carnival calendar with a single nighttime parade. Now there is a huge riverfront Lundi Gras festival and other local events in the area. It's also a bigger parade day, now featuring one of the Superkrewes, Orpheus with co-founder Harry Connick, Jr. and other celebrity riders.
Carnival means farewell to meat, before the beginning of Lent. It's all related to the Roman Catholic calendar, as evidenced by the Catholic-centric location of Mardi Gras celebrations worldwide.
The moon calculations that determine the day of Easter also determine Mardi Gras (Day) and Ash Wednesday. So the last day of Carnival varies but the first day is always January 6th which is Epiphany, the last day of the 12 days of Christmas, the day celebrating the arrival of the Magi at the the Nativity. Carnival season is full of parades, balls, parties, festivals, and King Cakes. Carnival is also King Cake season. The major parades in New Orleans are set for the two weeks before and including Mardi Gras (Day).
In New Orleans, Mardi Gras (Day) is now celebrated with two major parades, Zulu and Rex, which boasts the King of Carnival, followed by a long, long string of "truck floats", each individually created by organizations, friends and/or family with riders of all ages.
But parades are just the beginning. You can have a blast in New Orleans without going to the parade route. Creative costumes, mini parades, organized and impromptu entertainment, walking clubs, dancing, laughter, street theater and more is everywhere, especially on the Canal Street neutral-ground, the French Quarter, and in the Marigny neighborhood. Bourbon Street is so crowded on Mardi Gras nowadays, you really can't enjoy it. The famous Mardi Gras Indian tribes are out and about in various neighborhoods.
New Orleans Mardi Gras is an indescribable scene and an incredible atmosphere - You just have to be there. It's a remarkable day, especially when you consider just about everywhere else in the country, it's just a Tuesday and everyone has to go to work and school.
Mardi Gras and Carnival, the season, ends at midnight. Amazingly, the New Orleans Police Department clears the crowded Bourbon Street, with the help of mounted police, at midnight declaring "Mardi Gras is over".
Mardi Gras is also celebrated with parades, balls, and such at other town in Louisiana and several other Gulf Coast locations, like Mobile, Alabama which claims to have held the first Mardi Gras in the New World. The first Mardi Gras parade in New Orleans was formed by young men from Mobile. There are also several Louisiana site of Cajun Mardi Gras, which is an entirely different type of tradition involving often-drunken horseback rides in the country to gather chickens, vegetables, etc. for a community celebration.