Mardi Gras is just a popular name in French-speaking countries for the last day of the
Carnival season. It literally translates to Fat Tuesday. In England the Day is known as Shrove Tuesday, and celebrated with the eating of Pancakes.
Carnival is an unofficial festival that is an offshoot of the Catholic year. In preparation for lent when the eating of meat, eggs and fat of any kind was forbidden, all these rich foods that remained were eaten in a series of feasts which gradually evolved into Carnival.
Carnival also gathered in other traditions such as a reversal of roles (men acting as women, masters waiting on their servants,), the wearing of fancy dress, and the idea that no-one could refuse to participate in the fun.
The final day of carnival, (Mardi Gras), the day before Ash Wednesday, is the peak of Carnival festivities in most Carnival Countries. (In Carnival-celebrating parts of Germany, Rose Monday is the chief day). Officially, behaviour on this day should not be sinful, but good clean fun. Just as behaviour at Christmas should not be sinful. However Carnival was generally more tolerated by the Church than encouraged, and excesses were always condemned. Most protestant countries outlawed carnival at the Reformation. In England, remnants like pancake eating and sporting events survived, but the partying went.
Nice, france
Venice
Rio
Carnival can be very enjoyable community fun.
However, in many places other groups have stepped in to purloin the idea of Carnival. In Rio, secular groups in the City council have used Carnival to promote condoms. Many gay groups have taken the idea of "Carnival" for their own celebration parades, particularly in Australia. In the UK west Indian "carnivals" are held, divorced from the Church year in Summer.