Weekdays in Portuguese are called dias úteis (literally 'useful days' - though everyone knows weekends are always more useful) and rather than having names based on pagan gods or planets like the rest of the Romance languages as well as in Germanic languages, the days of the week are numbered. This apparently follows the Latin terms for Easter week. Sunday, the first day of the week, is dies Domenica (literally Lord's Day) in Latin or Domingo in Portuguese. The next day, Monday, becomes the "second day of rest" (again, in observance of Easter week) and therefore Segunda-feira. The word feira in Portuguese means 'fair' and the closely related word férias means 'vacation'. I think it is a bit ironic that the dias úteis would refer to the days we should not work.
Portuguese - English
Fim de semana = Weekend
Domingo = Sunday
Sábado = Saturday
Dias úteis (literal translation)= Weekdays
Segunda-feira (Second fair) = Monday
Terça-feira (Third fair) = Tuesday
Quarta-feira (Fourth fair) = Wednesday
Quinta-feira (Fifth fair) = Thursday
Sexta-feira (Sixth fair) = Friday
I suppose I should go out and take advantage of the rest of this useful day.