The seven-day week structure comes from ancient Mesopotamian and Babylonian calendars, which assigned days to the seven visible celestial bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn).

Monday: Named after the Moon, from Old English Mōnandæg (“Moon’s day”). The Moon was associated with lunar deities in various mythologies.

Tuesday: Named after the Norse god Týr (or Tiw in Old English), linked to the planet Mars in Roman tradition (from Latin dies Martis, “Mars’ day”). Týr was a war god, equated with Mars.

Wednesday: Named after the Norse god Odin (or Woden in Old English), associated with the planet Mercury (from Latin dies Mercurii, “Mercury’s day”). Odin was a god of wisdom, like Mercury.

Thursday: Named after the Norse god Thor, linked to the planet Jupiter (from Latin dies Iovis, “Jupiter’s day”). Thor, the thunder god, was equated with Jupiter, the god of thunder and the sky.

Friday: Named after the Norse goddess Frigg (or Freya), associated with the planet Venus (from Latin dies Veneris, “Venus’ day”). Frigg was a goddess of love and beauty, akin to Venus.

Saturday: Directly named after the planet Saturn, from Latin dies Saturni (“Saturn’s day”), with no Norse equivalent. –

Sunday: Named after the Sun, from Old English Sunnandæg (“Sun’s day”), tied to solar worship and the Roman/Latin dies Solis.

Roman naming conventions were adapted by Germanic tribes, who substituted their gods for Roman ones (e.g., Thor for Jupiter). Moons, like Earth’s Moon, directly influenced Monday, but other days are tied to planets or deities, not moons of other planets. This system spread throughout Europe and persists in English and related languages.

Some cultures, like Portuguese, retain more direct planetary names (e.g., segunda-feira for “second day” instead of “Moon’s day”).