Carnival is one of the oldest continuously-celebrated festivals in the Western world, dating back to ancient times. The cherub oxen parade used to represent the opening of the Egyptian carnival, which lasted for one week. Carnival was celebrated in ancient Greece also. Greek carnival festivities were dedicated to Dionysus, the wine god. Carnival has represented the end of winter from the very earliest celebrations.
In ancient Rome there were two holidays announcing the arrival of spring: the Saturnalia and the Lupercalia. The latter was held on the fifteenth day of February and was organized for the first time by Remus in honor of the Wolf who had raised them. In those days virtually every excess was allowed even to the common people, or plebes, as they were called.
Carnival, as we now know it, is without any doubt a product of the Middle Ages and is directly connected to Easter, which always occurs on the Sunday after the first full moon in spring. From Easter we subtract six weeks (five of which are part of Lent) and the week prior to that is the week when Carnival is celebrated. The week culminates in the so called "fat" days, from Thursday through Tuesday before Ash Wednesday.
The word Carnival is thought to be derived from medieval Latin carnem levare, that is "to remove the meat" from the diet, observing the Catholic prohibition to eat meat during Lent.
From 1400 on, Carnival went under a series of attacks and ultimately became Christianized by clerics such as Savonarola. Both the Counter-reformation and the Catholic Church tried to eliminate the holiday which was felt to be too Pagan in nature.
Carnival reached its artistic peak during the Renaissance. From January to March very sumptuous shows and majestic parties were held. In Florence, Lorenzo de' Medici was an exceptional carnival director. He ordered the preparation of long parades with floats and people in costumes. With the birth of the "Comedy of Art," these costumes became known as the "Masks" (Pulcinella, Arlecchino, Pantalone, Colombina, etc.). Carnival started to decline in the second half of 1800 and only after World War II has it come back to be once again the craziest holiday of the year.
Carnival in Italy 
Venezia
The most famous Italian Carnival
Viareggio
The most spectacular Carnival
Ivrea
Historic Carnival not to miss
Carnival all around the world
Rio de Janeiro
The most famous one in the world
Nizza
The French Mardi Gras
New Orleans
The American Mardi Gras