Greco-Roman tradition names the days after the classical planets of Hellenistic astrology, Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn; each planet having been named after an ancient Roman / Greek god. This follows through with the majority of romance languages preserving the Latin names with celtic languages also adopting the Latin names early on.
Germanic people adapted the naming system from the Romans but substituting with Germanic deities with the exception of Saturday for some Nordic languages where it has no reference to a god but means " washing-day". English, and most Germanic languages, preserved the association of Sunday to the sun whereas many other European and romance languages have changed it to the equivalent of "the Lord's day".
Origin: The first day of the week, and most obviously named, is Sunday named after the Greek god Helios who personifies the sun. Helios is often depicted with a radiant crown and driving a horse-drawn chariot through the sky, he is a sibling to Selene (moon) and Eos (dawn).
Origin: From the Greek goddess Selene who represents and is the personification the moon, her Roman equivalent being Luna. The English name for Monday has been derived from Latin with the in Old English word literally meaning "moon's day", being derived from the Latin.
Origin: Identifying with the planet and Roman god Mars, Greek equivalent being Ares. The name for Tuesday in both Japanese η«ζζ₯ (η«ζ for Mars) and Korean νμμΌ (literally meaning fire day) also refer to the planet Mars but are unrelated to the god.
Origin: Nicknamed as Hump Day, Wednesday got its name from the Roman god and planet Mercury. Also known as Hermes in Greek mythology, the planet Mercury was named after the speedy messenger god as it moves across the sky faster than any other planet.
Origin:The Roman god Jupiter, associated with the planet and the personification of thunder, is identified with the Norse god Thor, leading to the origination of Thursday from Thor's day.
Origin: Frigg, the goddess of marriage from Norse mythology and equivalent Venus, goddess of love from Roman mythology, are associated with Friday. The Norse name for the planet Venus was Friggjarstjarna, "Frigg's star".
Origin: Named after the Roman god Saturn, who was the god of time. the Roman name remained since none of the Germanic gods were considered to be a counterpart. In Japanese, Saturday is εζζ₯ meaning soil day, εζ for saturn, literally meaning soil star.