Originally a pagan festival, today's Carnival is held from the day following Epiphany until Shrove Tuesday, the last day before Lent.
Everyone in the parade is in disguise, masked or made-up. Under the guise of anonymity, they blend into the crowd and while hidden behind their masks do what is normally forbidden.
The established order and allocation of roles is turned upside down (master/slave, men/women) and social conventions and rules are altered, disrupted and forgotten.
For a few hours, all the constraints of the rest of the year are rejected and the world is turned upside down. 
Parades of floral and fantastical floats are accompanied by orchestras and fanfares during the most famous carnivals: Rio with its samba rhythm, Venice with its magnificent costumes, Binche, Dunkerque with its famous giants, and the Diablada (Dance of the Devils) in Mexico.
Carnivals are periods of disruption during which the rules of normal life are suspended. They offer people the chance to protest as well as rejoice and enjoy their freedom.
Everything goes back to normal the next day.