Before the day of the Sun, we have the day of the darksun Saturn, the other brother, the other son of the creator God, the lesser light, but greatest light in the darkness, Saturn aka Loki.
Jacob Grimm, in his 1835 treatise Teutonic Mythology, gives a solid explanation why Saturday is named essentially for Loki. While the other days are clear translations from the Roman days of the week into their Germanic pagan equivalents, Saturday does not fit the pattern. Every other weekday is named for a god or goddess from the Norse mythology: Sunday/Sol, Monday/Mani, Tuesday/Tyr, Wednesday/Odin, Thursday/Thor, and Friday/Freya. Grimm asserts that the Saturday does not retain the name of the Roman Saturn, but is named for Sæter (“insidiator” or “one who lies in ambush”), a name that he connects with Loki and suports with Anglo-Saxon and Old Norse sources.