Why Days Are Named After Planets and Gods

The seven day names encode an ancient astronomical idea: each day is “ruled” by one of the seven classical planets — the wandering lights visible to the naked eye. Rome named the days for the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, in an order derived from astrologers assigning planets to the hours of each day. That planetary week spread across the empire and beyond, and its fingerprints remain on nearly every European language.

Romance languages kept the Roman gods almost intact. In French, Tuesday through Friday are mardi (Mars), mercredi (Mercury), jeudi (Jupiter), and vendredi (Venus); Spanish and Italian match closely. Sunday and Saturday were later reshaped by Christianity — dimanche and domingo come from dies Dominica, “the Lord’s day,” while Saturday in Spanish, sábado, descends from the Sabbath.

English took a different route: Germanic peoples translated the Roman gods into their own. Mars the war god became Tiw (Tuesday); Mercury was matched with Woden/Odin (Wednesday); Jupiter the thunderer became Thor (Thursday); Venus was rendered as the goddess Frigg (Friday). Saturn alone kept his Roman name — Saturday — while Sunday and Monday translate the sun and moon directly. Every English week is thus a hybrid: five Norse-Germanic gods bookended by two celestial bodies and one Roman holdout.

Other cultures rejected the gods entirely. Portuguese, shaped by early Church reform, numbers its weekdays — segunda-feira through sexta-feira, “second market-day” to “sixth.” Modern Greek, Hebrew, and Arabic largely count as well: Arabic’s Sunday is al-Ahad, “the first,” and Thursday al-Khamis, “the fifth,” with Friday as al-Jumu’ah, the day of gathering. Chinese numbers Monday through Saturday and reserves a special word for Sunday. Japanese, strikingly, preserves the planetary system — its days name the sun, moon, fire, water, wood, metal, and earth, the East Asian five elements matched to the same seven classical planets.

So a Thursday meeting is Thor’s day in London, Jupiter’s day in Madrid, “the fifth” in Cairo, and the wood-star’s day in Tokyo — one shared rhythm wearing a dozen mythologies. The world clocks at itsdately.com label each city’s current weekday in real time, whichever tradition named it.

The planetary order itself — why Sunday follows Saturday rather than sitting elsewhere — falls out of ancient astrology’s arithmetic. Astrologers ranked the seven planets from slowest to fastest (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon) and assigned them to the 24 hours of each day in endless rotation. Whichever planet ruled a day’s first hour named the day — and since 24 hours advances the sequence by three places, the day-rulers emerge in the familiar Saturday, Sunday, Monday order. The week’s sequence is, quite literally, a remainder: 24 mod 7.

The names carry one more fossil: which day gets the honor of ending the week. Cultures that begin on Sunday treat Saturday as the seventh day — matching the Sabbath of the Hebrew calendar — while ISO’s Monday start crowns Sunday as day seven, aligning the “weekend” as a true end. The very word “weekend” is younger than it feels, spreading with industrial labor reforms in the nineteenth and twentieth centuries as the five-day work week emerged; before that, the days had gods, but workers had only the one.

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Further reading: Encyclopaedia Britannica — Calendar For readers asking “Why Days Are Named After Planets and Gods”, the answer comes down to the specific factors covered above.