The Celestial Roots of Our Calendar Week

Our solar system is made up of an impressive bunch of planets. Starting closest to the Sun, there’s Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, still hanging on in spirit if not by definition. At one point in history, that list looked a bit different from the one we use today. Introduced by the Sumerians, accepted by the Babylonians, and eventually embraced by the Romans, the known celestial bodies—including the Sun and Moon, which were considered planets at the time—were used to name the days of the week. To be fair, the definition of a planet then was broader than it is today, allowing for those inclusions.

Image credit: Pluto’s Orbit

The literal word planet evolved from the ancient Greek word planētēs, derived from the verb planasthai, meaning "to wander." Observing the visible celestial bodies move independently from the settled starry background, it seemed as though they wandered about the Earth. This loose definition was eventually refined by 16th and 17th century astronomers, including Nicolaus Copernicus, who proposed the heliocentric model stating that the Sun, not Earth, is at the center of the solar system. With developing studies and the use of the telescope, the Sun came to be understood as a star, and the Moon as merely a satellite of the Earth. This understanding prompted a change in the definition of a planet. Today, the International Astronomical Union (IAU) defines a planet based on three preconditions:

• It must orbit a star (in our cosmic neighborhood, the Sun).

• It must be big enough to have enough gravity to force it into a spherical shape.

• It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

Still, our Earth-centered perspective produced the names for each day of the week. The seven-day week system named days after a Roman god, with Germanic and Norse cultures later adapting the names to their own deities and language structures, while keeping the planetary references. Before you continue, take a moment to speculate how each of the seven day names correlates with the planets, then check your assumptions below!

Image credit: Pluto’s Orbit

Sunday

Perhaps the most obvious, Sunday was named after our favorite star, the Sun. Sunday was designated as the first day of the week, though this clashed with Jewish tradition, which observes the Sabbath on the seventh day. The Spanish name Domingo comes from the Latin phrase dies Dominicus, meaning “the Lord’s day.” This Latin phrase inspired both the Spanish name Domingo and the Old English name Sunnandæg, which means “day of the Sun.”

Monday

Named after our one and only Moon, the word Monday comes from the Old English word Monandæg, meaning “Moon’s day.” Traced back to the Latin phrase dies Lunae, meaning “day of the Moon,” the Spanish name lunes was derived from it.

Tuesday

Tuesday comes from the Germanic and Norse god of war, Tiw. Tiw was comparable to the Roman god of war, Mars, after whom the planet Mars was named. The Old English word Tiwesdæg means “day of Tiw,” while the Latin phrase dies Martis means “day of Mars.” The Spanish name for Tuesday, Martes, was derived from this Latin phrase.

Wednesday

Tiw was a son of Odin, or Woden, the supreme deity who inspired the name Wednesday. Odin served as a traveler between worlds, including Hell, and was recognized as a messenger. Some gods are known across different mythologies, in this case Germanic, Norse, and Roman, and were considered to be the same figure, only with different names and associated tales. The traveling and communicative traits were shared with the Roman god Mercury, whose name was also given to the closest planet to the Sun. The Germanic peoples adopted the Old English word Wodensdæg, which means “Woden’s Day.” Originating from the Latin Dies Mercurii, meaning “Day of Mercury,” miércoles became the Spanish name for Wednesday.

Thursday

Similarly, Thursday originates from another of Odin's sons, named Thor, the god of thunder. The Germanic peoples saw Thor as comparable to Jupiter, the Roman king of the gods, associated with thunder, sky, and lightning, and the namesake of the largest planet in our solar system. The Old English word Þūnresdæg means “Thor’s Day,” and the Latin Dies Jovis means “Day of Jupiter,” which gave rise to the Spanish name jueves.

Friday

The only day and planet named after a female god, Friday, was inspired by Frigg, the goddess of beauty. Also the wife of Odin, Frigg represented love and femininity, and can be interpreted as overlapping with or separate from Freya, another Norse goddess. The Roman counterpart was Venus, goddess of beauty, who also gave her name to the second planet from the Sun. Friday was named after Frigg, with the Old English Frigedaeg translates to “Frigg’s/Freya’s Day,” and the Spanish name viernes comes from the Latin phrase dies Veneris, meaning “day of Venus.”

Saturday

Saturday was named after the Roman god of agriculture, time, and wealth, Saturn. A ruler during the mythic Golden Age, Saturn remained the only Roman god whose name was not replaced with a Norse equivalent. The Old English Sæterdæg, meaning “Day of Saturn,” was derived from the Latin dies Saturni, or “day of Saturn,” and has remained unchanged. However, you may notice the Spanish word for Saturday, sábado, does not resemble the Latin origin. Instead, sábado comes from the Hebrew Sabbath, meaning “the seventh day,” a day of rest.

Some languages today have clearer translations, though all ultimately trace back to this original planetary system based on the seven observable celestial bodies. There is a rich history to explore in the naming of months and other celestial bodies, including stars and constellations, but it’s fascinating to start with the days of the week that shape our routine. Perhaps this breakdown has made you rethink your favorite day, or maybe being named after the Moon makes Monday no more amusing. Either way, it’s an interesting piece of history rooted in our daily lives, showing how foundational concepts continue to influence us through time.

Works Cited

“Definition of PLANET.” Www.merriam-Webster.com, www.merriam-webster.com/dictionary/planet.

Name Explain. “How Did the Days of the Week Get Their Names?” YouTube, 21 Sept. 2018, www.youtube.com/watch?v=0G0XiiTyeeQ. Accessed 26 May 2025.

NASA. “What Is a Planet? - NASA Science.” Science.nasa.gov, Nasa, 2024, science.nasa.gov/solar-system/planets/what-is-a-planet/.

StarTalk. “The Reason We Have 7 Days in a Week.” YouTube, 11 July 2024, www.youtube.com/watch?v=JZG1pp1K1QQ. Accessed 19 Mar. 2025.

“The Copernican Model: A Sun-Centered Solar System.” Rochester.edu, 2000, www.pas.rochester.edu/~blackman/ast104/copernican9.html.

“Week | Origin, History, & Facts.” Encyclopedia Britannica, www.britannica.com/science/week.