The Latin Names for Days of the Week

Romans named the days of the week after the seven known planets–or rather, celestial bodies—which had been named after Roman gods: Sol, Luna, Mars, Mercury, Jove (Jupiter), Venus, and Saturn. As used in the Roman calendar, the gods' names were in the genitive singular case, which meant each day was a day "of" or "assigned to" a certain god.

• dies Solis, "day of the Sun"
• dies Lunae, "day of the Moon"
• dies Martis, "day of Mars" (Roman god of war)
• dies Mercurii, "day of Mercury" (Roman messenger of the gods and god of commerce, travel, thievery, eloquence, and science.) 
• dies Iovis, "day of Jupiter" (Roman god who created thunder and lightning; patron of the  Roman state) 
• dies Veneris, "day of Venus" (Roman goddess of love and beauty)
• dies Saturni, "day of Saturn" (Roman god of agriculture)

Latin and Modern Romance Languages

All of the Romance languages–French, Spanish, Portuguese, Italian, Catalan, and others—were derived from Latin. The development of those languages over the last 2,000 years has been traced using ancient documents, but even without looking at those documents, the modern-day names of the week have clear similarities to the Latin terms. Even the Latin word for "days" (dies) is derived from the Latin "from the gods" (deus, diis ablative plural), and it too is reflected in the endings of the Romance language day terms ("di" or "es").

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Origins of the Seven-Planet Week

Although the names of the week used by modern languages don't refer to gods that modern people worship, the Roman names definitely did name the days after the celestial bodies associated with particular gods—and so did other ancient calendars.

The modern seven-day week with days named after gods associated with celestial bodies, is likely to have originated in Mesopotamia between the 8th and 6th centuries BCE. The lunar-based Babylonian month had four seven-day periods, with one or two extra days to account for the movements of the moon. The seven days were (probably) named for the seven known major celestial bodies, or rather for their most important deities associated with those bodies. That calendar was communicated to the Hebrews during the Judean exile in Babylon (586–537 BCE), who were forced to use the imperial calendar of Nebuchadnezzar and adopted it for their own use after they returned to Jerusalem.

There's no direct evidence for the use of celestial bodies as name days in Babylonia—but there is in the Judean calendar. The seventh day is called Shabbat in the Hebrew bible—the Aramaic term is "shabta" and in English "Sabbath." All of those terms are derived from the Babylonian word "shabbatu," originally associated with the full moon. All of the Indo-European languages use some form of the word to refer to Saturday or Sunday; the Babylonian sun god was named Shamash.

Adoption of the Seven Day Planetary Week

The Greeks adopted the calendar from the Babylonians, but the rest of the Mediterranean region and beyond did not adopt the seven day week until the first century CE. That spread into the hinterlands of the Roman empire is attributed to the Jewish diaspora, when the Jewish people left Israel for the far-flung elements of the Roman empire after the Second Temple destruction in 70 CE.

The Romans didn't borrow directly from the Babylonians, they emulated the Greeks, who did. Graffiti in Pompeii, destroyed by the eruption of Vesuvius in 79 CE, includes references to the days of the week named by a planetary god. But in general, the seven-day week was not widely used until the Roman Emperor Constantine the Great (306–337 CE) introduced the seven-day week into the Julian calendar. The early Christian church leaders were appalled at the use of pagan gods for names and did their best to replace them with numbers, but with no long-lasting success. 

-Edited by Carly Silver

Sources and Further Reading

• Falk, Michael. "Astronomical Names for the Days of the Week." Journal of the Royal Astronomical Society of Canada 93:122–133
• Ker, James. "'Nundinae': The Culture of the Roman Week." Phoenix 64.3/4 (2010): 360–85. Print.
• MacMullen, Ramsay. "Market-Days in the Roman Empire." Phoenix 24.4 (1970): 333–41. Print.
• Oppenheim, A. L. "The Neo-Babylonian Week Again." Bulletin of the American Schools of Oriental Research 97 (1945): 27–29. Print.
• Ross, Kelley. "The Days of the Week." The Proceedings of the Friesian School, 2015.
• Stern, Sacha. "The Babylonian Calendar at Elephantine." Zeitschrift für Papyrologie und Epigraphik 130 (2000): 159–71. Print.