Generally related to the Christian festival of Mardi Gras, Carnivals are festive events that are relatively wide-spread in Europe and South America.
The word "Carnival" can be derived from carne (meat or flesh) and the word for swallow, given that great quantities of meat are consumed during Carnival to compensate for abstinence imposed during the Fasting season. Other authors claim that the word comes from caro vale, i.e. goodbye to meat. But all these etymologies are quite imaginary. Linguists regard only the Italian origins, being carnevale, composed of carn (meat) and levare (lift, remove, dare).
During the period of Carnival, the city inhabitants go out disguised (with masks or make-up) and meet to sing and dance in the streets, throwing confetti, sometimes participating in parades.
According to the religious calendar, Carnival begins with the celebration of the Epiphany (January 6th), marking the end of the Christmas holidays, and ends with Mardi Gras (Fat Tuesday), on the day before the Fasting season begins.
In Germany, Carnival begins officially on November 11th at 11:11 a.m. and ends the following year on Mardi Gras. In Greece it's called Apokriá, ending on the Monday before Mardi Gras.
Colourful cloths in all varieties, late evening masked balls, and parades on the Sunday and Tuesday before the Fasting season begins are the main amusements offered during Carnival. The Carnival in Venice and in the temperate climate zones in general are the brightest and most celebrated events.
Source: Wikipedia (30/11/2009), own translation
Some interesting links
Carnival in Cologne (Germany)
Carnival de Mainz (Germany, in German)
Carnival in Venice (Italy)
Carnival in Rio (Brazil)
Mardi gras (Carnival in New Orleans, USA)
Carnival in Basel (Switzerland)
Foutapadia (Fouta Djallon, Guinea, in French)