The original purpose of the Carnival was to rid yourself of all the excesses you might have - some of the extra "treats" you might have left over from Christmas. Now is the time to have a party and consume all that you can, for on Ash Wednesday we begin 40 days of penitential observance as we prepare for Easter Sunday and the Resurrection of our Lord and Savior.
The former tradition (still practiced among Eastern Orthodoxy) was to rid your household of all meat products. No beef, no chicken, no fish, no butter, no milk, no cheese - in short, it was a very vegan holiday. There still are good reasons, healthy reasons, to give up all meat products for 40 days per year. The current law in place for Latin Rite Catholics is fasting and complete abstinence from meat (fish is allowed) on Ash Wednesday and all the Fridays of Lent. The fast allows for one full meal and two smaller meals, which if put together do not equal one full meal - and no snacking in between. Sundays remain solemnities, and therefore fasting and abstinence is not required on Sundays (though you still could do so). Those who are required to participate in the fasting are those who have reached 18 years old and not yet reached 60. There is no age on abstinence, all Catholics are required, regardless of age, to participate in abstinence (unless there is some medical necessity).
Then there's the "What You Gave Up For Lent" tradition. Again, this is not an official mandate of the Catholic Church - but it is very popular among Catholics and other Christians too. The purpose is to give up something, easy or hard, that you would normally partake in on a daily basis. When you would have partaken in whatever that is, you think upon the penance Jesus did for 40 days in the desert before beginning His public ministry and/or meditate on something from Passion Week, where He suffered immensely for our sins, eventually to be wholly humiliated and crucified. Lent is a time of "putting away the flesh..." a time of "carnival."
from medieval Latin carnelevamen, carnelevarium ‘Shrovetide,’ from Latin caro, carn- ‘flesh’ + levare ‘put away.
The problem we have is that Mardi Gras has become a secular "holiday" and many people engage in excess just for the sake of being excessive. Unfortunately, this may (and often does) include other sinful acts. While much of what goes on in places like New Orleans is not done by Catholics, many Catholics are drawn in by the excitement and temptations of the excesses, so what started out as a means to rid ones self of excess, it has become a day which invites excess where there may not have been any to begin with - it becomes an excuse to be sinful.