Which day of the week has been derived from a planets name?

"Saturday" is derived (skipping several steps along the way) from the Latin Saturni dies, meaning Saturn's day -- referring to the planet, not the mythological figure. Most of the other English days of the week, however, are named after gods -- albeit Norse ones. Tuesday comes from Týr, Wednesday from Odin (Wodin), Thursday from Thor and Friday from Frigg. Sunday and Monday are named after the sun and the moon (not planets), respectively.

Interestingly, all days of the week are named after heavenly bodies in Latin; some of them just overlap with names of Roman gods. For example, Tuesday was dies Marti, referring to the planet Mars, not the God of War. When these names were being adapted to Germanic languages, however, Norse gods analogous to the gods for whom the planets referenced in the Latin weekdays were named were substituted in. Thus, Týr, the Norse god of war, replaced Mars -- even though dies Marti referred to the planet, not the God -- to create Tuesday. Similar things happened with Jupiter and Thor, and with Venus and Frigg. Sunday and Monday, though, were translated correctly; they always referred to the sun and the moon.