The origin of the names of the days of the week in Portuguese

But why? Why??? In Spanish they have names related to astronomical objects (Moon-Day, Mars-Day, Mercury?-Day, Jupiter-Day, Venus?-Day) Why in Portuguese it's just 2-Day, 3-Day, 4-Day, 5-Day, 6-Day?

Graças à Deus, amanhã chega sexta-feira.

Não se sabe ao certo a origem de "feira" nos dias da semana em Português.