Since I am a Catholic, I can explain what Mardi Gras is all about.
It is the last night of revelry before Ash Wednesday, which is a day of fasting and no meat allowed.
Following that are 40 days of Lent, which is considered a time of fasting and prayer. On all Fridays of Lent, Catholics may not eat meat. At Mass the word "Alleluia" may not be said, and the priests wear dark purple, and there are no flowers. The whole point of Lent is self-denial, and penance, to prepare for Easter, which is a celebration of the resurrection of Christ, proving that even death can be conquered.
The reason Mardi Gras is celebrated in New Orleans is because the city was founded by the French, who are Catholic. Mardi Gras is a last time that people can go crazy before Lent.