Mardi Gras fancy dress costumes form a very important part of the carnival celebrations that take place at this time of year. The whole point of Mardi Gras is to have one last load of fun, before Lent begins, and a carnival fancy dress outfit is nothing if not fun.
The phrase "Mardi Gras" is French for "Fat Tuesday" and basically it refers to the age old tradition of eating richer and fattier foods before fasting for Lent, which lasts forty days and begins the day after, on Ash Wednesday.