Carnevale (or Carnivale, or Carnavale, alternate spellings you may find), is a festival which can trace its roots back to the Roman Saturnalia, a festival held in mid-December to honor the god Saturn with feasting, gift giving and role reversal. As often happened with such festivals, Catholics found a way to work the festival into their own liturgical year.
One commonly accepted derivation of the word "carnival" is the Latin "carne vale" or "farewell to meat." Carnival was the final hurrah as winter headed towards spring, and the long Lenten season of fasting and abstinence. It was, perhaps, not only a last chance to indulge the passions of the flesh, but an opportunity to consume any meat which had been put up for winter that might not stay fresh enough for consumption until spring brought the end of Lent and Easter.
Because of its ties to the liturgical calendar, Carnivale varies in length. It can be considered to begin with the feast of Epiphany (the 12th day of Christmas) on January 6, and always ends on Shrove Tuesday, the day before Ash Wednesday, the first day of Lent. The Lenten season is the 40 days before Easter, which falls on the first Sunday after the first full moon after the vernal equinox.
If you've never heard of Shrove Tuesday, perhaps you've heard of "Fat Tuesday" -- also known in French as Mardi Gras, or in Italian as Martedi Grasso.
Carnivale is celebrated with enthusiasm around the world; in the US the celebration in New Orleans is best known. In Brazil, Carnivale in Rio is famous for its wild abandon. Many cities in Italy have maintained or revived their traditions of Carnivale; especially well-known among them are Venice and Florence.
While Carnivale has a rich history and ancient roots in Italy, the festival remains a vital and contemporary celebration. A simple web search will reveal many images of celebrants in both historical and contemporary costumes. One of the common elements found throughout the tradition has remained the Carnivale mask.