Where do the names of the days of the week come from? And why are there seven days in a week? The answer involves Hellenistic astrology, Roman gods and goddesses, and a takeover by Norse mythology.
The concept of the seven-day week was first recorded in the Babylonian calendar of ancient Mesopotamia, which is based on the 21st-century BCE Sumerian calendar. Each of the four phases of the moon (waxing, full, waning, new) lasts about seven days, and the seven-day period corresponds to the time it takes the moon to transition. Four weeks—a “moon,” or a “month”—is about the length of a complete moon cycle. A complete moon cycle lasts precisely 29.53 days, so one or two days were inserted at each set of four weeks until the new crescent moon signaled the beginning of a new month. Additionally, an extra month was sometimes inserted by royal decree to reconcile the lunar year with the solar year.
The Jewish seven-day week was a distinct tradition that emerged from the biblical account of the seven-day creation period. The Book of Genesis describes God creating the world in seven distinct days or time periods, resting on the seventh day. This became the basis for the seven-day week in Judea, with the seventh day reserved for a Sabbath, or day of rest.
The seven-day week gradually gained popularity with the Romans starting in the first century BCE. By the fourth century CE, this system had become dominant across the Roman Empire and had spread to India and China as well. The Romans assigned each day a name of one of the seven visible planets in the sky, all those that were known to humans at the time (including the sun and moon, which were considered planets). The planets had each been named after a Roman god or goddess.
In Hellenistic astrology, the planets were commonly listed in the order of fastest-orbiting to slowest-orbiting from the perspective of earth: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn. This is known as the Chaldean order. Astrologers believed that a different planet ruled over each hour of the day, and the planetary hours followed the Chaldean order. The days of the week were assigned every other name in this order to correspond with the planetary hour that begins each day at sunrise. Through this somewhat complicated astrological system, the days of the week were given the following names:
Monday (dies Lunae) was the moon’s day. Diana was the goddess of the moon.
Tuesday (dies Martis) belonged to Mars, the god of war.
Wednesday (dies Mercurii) was Mercury’s, the god of commerce, wealth, translating and interpreting, travel, and thievery. Mercury is known as the messenger god.
Thursday (dies Jovis) was for Jove, an earlier name for Jupiter. As the king of the Roman pantheon, Jove was the god of the sky and of thunder and lightning.
Friday (dies Veneris) was named for Venus, the goddess of beauty, fertility, and love.
Saturday (dies Saturni) was Saturn’s day. Saturn was the god of agriculture, liberation, and time.
Sunday (dies Solis) was for the sun. Apollo was the god of the sun.
If you speak a Romance language, the Latin names might look familiar to you. With the exception of Saturday and Sunday, which in many languages were changed to “Sabbath” and “the Lord’s Day,” the Roman names of the days of the week were preserved in Latin-based languages.
The Romans could have just named these days “Day 1,” “Day 2,” “Day 2,” and so on—in fact, Portuguese does just this for weekdays. Many non-Romance languages, like Russian and Hebrew, number each day of the week starting on either Sunday or Monday. But many societies under the umbrella of the Roman Empire chose to honor the celestial bodies that represented their deities.
The Romans occupied the island of Britain from 43 to 410 CE, and the seven-day calendar week with its planetary associations spread to the Old-English-speaking inhabitants. The English days of the week correspond in meaning to the Latin days of the week, but they are etymologically distinct. The days of the week in English bear the names of Anglo-Saxon deities, gods and goddesses that originated in Germanic and Norse mythology.
(As a quick primer on the Old English names given in parentheses, the æ character is called ash and is pronounced like the a in ash. The g at the end of each word is pronounced like a y.)
Monday (Monandæg) is still the moon’s day, from Old Norse máni, meaning “moon.”
Tuesday (Tiwesdæg) is Tiw’s day, named for the Norse god of war and combat. Notice that Tuesday is named after the god of war in both languages—in Latin, this god is Mars, and in English, this god is Tiw.
Wednesday (Wodnesdæg) is named for Woden, the chief Anglo-Saxon god in charge of wisdom, healing, death, royalty, and the runic alphabet, among other things. Unlike the other namesakes of the days of the week, Woden does not have an obvious connection with his Roman counterpart, Mercury. However, upon a closer examination, both are shapeshifting gods who are associated with writing and with the dead. The possessive form of Woden in Old English was Wodnes, which accounts for the strange spelling of the word. In Norse mythology, this god is named Odin, whom you might know from The Avengers.
Thursday (Ðunresdæg) is Thor’s day! Thor, whom you definitely know from The Avengers, is the hammer-wielding Norse god of thunder and lightning. Like the Roman god Jove, he rules over the skies. The Anglo-Saxon version of the name was Ðunor (or Thunor in modern spelling).
Friday (Frigedæg) is named after Friga, the goddess of love, who corresponds to the Roman Venus. She is associated with the home, marriage, and children. She was seen as the mother of the earth and is the wife of Woden.
Saturday (Sæternesdæg) adopted the name of the Roman god Saturn. No substitution occurred, possibly because there was no corresponding Anglo-Saxon god.
Sunday (Sunnandæg) similarly honored the sun, from Old Norse sól.
The Roman seven-day week and names for the days of the week were thus given a distinct Anglo-Saxon character in Old English, an example of cultural diffusion and adaptation that continues to influence our language and worldview today.
Since many aspects of the calendar are tied to the lunar and solar cycles, it makes sense why the Romans named the days of the week after the celestial bodies in the solar system. The planets revolve and rotate in order and harmony, just like the days of the week provide order for our lives.
Bultrighini, Ilaria. “The Seven-Day Week in the Roman Empire and the Near East.” Accessed December 2, 2020. University College London Hebrew & Jewish Studies. https://www.ucl.ac.uk/hebrew-jewish/research/research-projects/calendars-late-antiquity-and-middle-ages-standardization-and-fixation-1.
Coolman, Robert. “Keeping Time: Origins of the Days of the Week.” May 7, 2014. Live Science. https://www.livescience.com/45432-days-of-the-week.html.
Online Etymology Dictionary. etymonline.com.
Johnson, Ben. “The Anglo-Saxon English Days of the Week.” Accessed December 2, 2020. Historic UK. https://www.historic-uk.com/CultureUK/Days-Of-The-Week/.
Koyfman, Steph. “The Secret Language of Weekdays.” January 25, 2019. Babbel. https://www.babbel.com/en/magazine/weekday-origins.
“Names of the Days of the Week.” Accessed December 2, 2020. Wikipedia. https://en.wikipedia.org/wiki/Names_of_the_days_of_the_week.
“Planetary Hours and Days.” 2018. Renaissance Astrology. https://www.renaissanceastrology.com/planetaryhoursarticle.html.