History of Rio de Janeiro

Guanabara Bay was reached by a Portuguese expedition under Florentine explorer Amerigo Vespucci, that included Portuguese explorer Gaspar de Lemos, on January 1, 1502; hence Rio de Janeiro, "January River." There is a legend that the mariners named the place thus because they thought the mouth of the bay was actually the mouth of a river, but no experienced sailor would make that mistake. At the time, river was the general word for any large body of water.

Source: Wikipedia — History of Rio de Janeiro (CC BY-SA 4.0)