Carnival

During the 18^th century, the Portuguese, settlers of Brazil, used to have a feast to mark the beginning of the period of quaresma.

Quaresma is the period of 40 days which goes from the Ash Wednesday to the Easter Sunday; because Catholics are supposed to follow some penitency during the Lent, the feast was a kind of pre-tension-reliever.
One of the penitencies during Lent is to avoid the consumption of meat; "carnevale", in ancient Italian, means exactly that: to stay away from meat ("carne" still means meat in modern Portuguese), and this would be the origin of the word Carnival.

The feast was popular among populars and the noblesse, which could break free from their obligations and throw small espheres of wax, containing perfumed water, against their peers.

This feast, called entrudo, was brought to Salvador when the Royal Family was forced to move to Brazil; the natives, short of perfumed water, quickly made an adaptation: they would use rotten eggs and dirty water.
Because of this mess, the entrudos were prohibited, and that feast was replaced by Carnival, a party which used to occur in Nice and Veneza at the same epoch as the entrudos; the European Carnival had people in costumes and fantasies, and the peak of the party, people paraded around the streets singing, dancing and joking.

Again, the Brazilian creativity came into place: they mixed up entrudo and Carnival, and created a celebration with fantasies, dance, and parades and, above all, the sense that the laws were temporarily laxed and everyone is allowed to have fun. So, the Brazilian Carnival was born.

Today, Carnival in Brazil is basically the same: a period of feast, when people may forget some social rules and just have fun. Evidence is that Carnival is referred to as folia, which means something like "mess", or something with no rules.
Most Brazilian cities have Carnival, either promoted by the governments or, spontaneously, by the people. If everything else lacks, people just meet in a large area, turn a loud sound on and start dancing and throwing objects against whoever is closer.
Officially, Carnival lasts from previous Friday through the Ash Wednesday; many people start early and finish late, though.