There is one major theme that binds together all the days of the week—all of them are named after gods. Furthermore, out of the seven days of the week, six of the days are named after Germanic and Norse gods.
Sunday is named after the Germanic sun goddess Sunna. Monday is named after the Germanic moon god, Máni. Tuesday is named after the Norse god of combat, Týr. Wednesday is named after Germanic god and soul-tender, Woden. Thursday is named after the Norse god of thunder, Thor. Finally, Friday is named after the Norse goddess of foresight and wisdom, Frigg.
This leaves us with Saturday, the only day of the week whose name cannot be traced back to either Germanic or Norse deities. It’s the only day of the week that retains its Roman origin in English and is named after the Roman god Saturn. As it stands, Saturday is “Saturn’s Day”, which is a far more noble name than we would have had if we had retained the old Norse designation—Saturday would instead be laugardagr or, literally, “washing-day”.