As with our months, the origins are from the Vikings and Romans.
MONDAY - from the Romans, meaning "moon's day."
TUESDAY - from the German meaning "Tyr's Day" - Tyr is a defender god in Viking mythology and the Germanic god of war and the sky.
WEDNESDAY- From Viking meaning "Woden's Day or Odin's Day ."
THURSDAY- From Viking meaning "Thor's Day."
FRIDAY- From "Frey's Day. " Frey was a male god who gave his sword to a mortal (Skirnir) who lost the sword - so Frey could not battle in Ragnarok (that was the final battle or twilight of the gods). Another possibility is that it comes from Frigg's day - she was the wife of Odin, the Queen of the heavens and the goddess of love and the household.
SATURDAY- From Roman meaning "Saturn's Day." He is the father of Jupiter, Neptune, Pluto, Ceres and Juno.
SUNDAY-From the Romans - meaning "Sun's Day."