Today I found out why we have a seven day week and the origins of each day’s name.

Two of the earliest known civilizations to use a seven day week were the Babylonians and the Jews. The Babylonians marked time with lunar months and it is thought by many scholars that this is why they chose a seven day week (though direct evidence of this being why they did this is scant). That being said, each lunar month was made up of several different cycles—on the first day, the first visible crescent appeared; on approximately the seventh, the waxing half-moon could be seen; on approximately the fourteenth, the full moon; on approximately the twenty-first, the waning half-moon; and on approximately the twenty-eighth, the last visible crescent. As you can see, each notable cycle is made up of about seven days, hence, the seven-day week.

You’ll notice I used the word “approximate” a lot in there. This is because the moon phases don’t line up perfectly with this schedule. As such, as far back as the 6th century BC (which incidentally is also around the time the Jews were captives in Babylon), the Babylonians would sometimes have three seven day weeks, followed by an 8-9 day week, presumably to re-synchronize the start and end of the weeks to match the phases of the moon.

In their normal seven day week, the Babylonians held the seventh day of each week as holy, much like the Jews did and still do. However, the Babylonians also held the day to be unlucky. Thus, similar to the Jews (but for a different reason- the unluckiness of the day), the seventh day had restrictions on certain activities to avoid dire consequences from the inherit unluckiness of the day. The final “seventh day” of the month for the Babylonians was a day of rest and worship.

By deistic decree, the Jews also followed a seven day cycle with the seventh day- the Sabbath- to be a day of rest and worship. In fact, the word “Sabbath” comes from the Hebrew “shabbath”, meaning “day of rest”, which in turn comes from the Hebrew “shabath”, meaning “he rested”- thus resting in homage to God resting on the seventh “day” after creating the universe. (Note: some biblical scholars believe the “day” here, in terms of six “days” to create the universe, one to rest, is more accurately translated as “period” or “interval” rather than a literal Earth day. This is perhaps not unlike the “40 days and 40 nights” Jewish saying being a non-literal ancient Jewish expression simply meaning “a really long time”.)

Unlike the Babylonians, where it appears they were attempting to follow the lunar cycles with their seven day week, it isn’t known why the Jews picked seven days, outside of Christians and Jews of course believing that it was by the decree of God.

Whatever the case, the Ancient Romans, during the Republic, did not use a seven day week, but rather went with eight days. One “eighth day” of every week was set aside as a shopping day where people would buy and sell things, particularly buying food supplies for the following week.

Rather than labeling the days of the week with actual names, at this time the Romans labeled them with letters, A-H. You might think from this that the “H” was always the shopping day, but this isn’t correct. You see, the calendar year did not divide evenly by eight. Thus, the day of the week that was the day to go shopping changed every year, but they still often referred to days based on its proximity to the shopping day.

For reasons not entirely clear, within a century after the introduction of the Julian Calendar was introduced in 46 BC, the eight day week started to diminish in popularity in favor of the seven day week. The full switch was not sudden, happening over centuries, and for a time, as the seven day week grew in popularity, both the seven and eight day weeks were used in Rome simultaneously. Finally, after the popularity of the eight day week diminished to almost nothing, Constantine, the first Christian Roman Emperor, made the seven day week official in AD 321. Due to the influence of both Rome and Christianity, this has stuck in most regions of the world ever since.

So now what about the origins of the names of the days of the week? Ancient Mesopotamian astrologers assigned each day the name of a god. The Greeks later called these days “theon hemerai”, or if that’s all Greek to you, “days of the Gods”. In a culture where days were consumed by religion, it’s only natural that the days of the week were made in homage to the gods thought to rule the lives of mortals. The days of the week follow the same trend as the months of the year, many of which (including January and March) are named after gods from several different pantheons.

The Romans, upon beginning to use the seven day week instead of the eight day, then adopted the names of the week to fit their own gods. The names of the week were then adopted by Germanic peoples. Despite Greek and Roman gods being the more popular and more well-known of the pantheons, it is largely the Germanic and Norse gods that have received the most credit and live on in the names of the days of the week today.

While different societies start the week on different days—usually Sunday or Monday—I’ll start with Monday, which was named for the moon. It could be translated as “Moon’s day”. This homage to the moon can be seen in several other languages as well. In Latin, it’s “dies lunae”, or “day of the moon”. In ancient Greek, “hemera selenes”, which means the same thing. In more modern languages, Monday is “lunes” in Spanish and “lundi” in French, both of which come from the root word for moon—”luna” and “lune” in each respective language.

Tuesday is the first to be named after a god. It was named for Tiu, or Twia, a lesser-known god of war and the sky from the English/Germanic pantheon. He is also associated with the Norse god Tyr, who was a defender god in Viking mythology. However, Tuesday does not translate the same in other languages. In Latin, it’s “dies Martis” or “Day of Mars” and in ancient Greek it’s “hemera Areos” or “day of Ares”. Both Mars and Ares were gods of war like Tyr and they lent their names to day of the week translations for other modern languages. Tuesday is “martes” in Spanish and “mardi” in French, both named for the Roman god Mars.

Wednesday can be translated as “Woden’s day”. Woden, associated with the Norse god Odin, was the chief god and leader of the wild hunt in Anglo-Saxon mythology. Directly translated, “woden” means “violently insane headship”, and does not put one in mind of the best of gods. Unlike the other days of the week, the gods named in the Latin and Greek days of the week – Mercury and Hermes — are not associated with violent leadership, but with travel, commerce, and theft. Both are messenger gods. It is for Mercury that Spanish and French decided to name Wednesday—”miercoles” and “mercredi” respectively.

Thursday is one of the easiest days to translate, meaning “Thor’s day”. Named for the Norse god of thunder and lightning. Thursday is also associated with Jupiter in Latin (“dies Jovis”) and Zeus in Greek (“hemera Dios”). All three gods are known for their storm-creating abilities, but while the English language took Thor as its god for Thursday, Spanish and French adopted Jupiter instead, naming Thursday “jueves” and “jeudi” which have roots in Jupiter.

Friday is associated with Freya, the Norse goddess of love, marriage, and fertility. The Latin, “dies Veneris”, and the Greek, “hemeres Aphrodite”, call upon the goddesses Venus and Aphrodite instead. The latter two goddesses are also patrons of love and beauty, and all three goddesses are called upon in womanly matters like fertility and childbirth. Following the trend of the other days, Spanish and French adopted Venus for Friday rather than Freya, naming their days “viernes” and “vendredi”.

Saturday in English derives from “Saturn’s day” which was taken from the Latin, “dies Saturni”. Saturn was a Roman god and, over different periods of time, associated with wealth, plenty, and time. The day in Spanish and French (“sabado” and “samedi” respectively) was named simply as it is the Jewish Sabbath- “sabado” deriving from the Latin “sabbatum”, meaning “Sabbath”, and “samedi” deriving from the Old French “samedi”, which in turn comes from the Latin “dies Sabbati”, meaning “Day of the Sabbath”.

Sunday is “Sun’s day”, translated in both Latin (“dies solis”) and Greek (“hemera helio”) as “day of the sun”. Interestingly, in Spanish and French (“domingo” and “dimanche”) it is more closely translated as “Lord’s day” or “Sabbath day”, pointing to more the Christian/Jewish God.

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Bonus Facts:

For a very brief time in France, the French abandoned the seven day week in favor of a ten day week, beginning in 1793 thanks to the new republican calendar developed in France at that time. This was abandoned nine years later when the Roman Catholic Church was reestablished in France. The official switch back to the seven day week happened on April 18, 1802- Easter Sunday.

The USSR also for a time (starting in 1929) abandoned the seven day week in favor of at first a five day week, then a six day week. This in turn was abandoned and the seven day week was re-established in 1940.

One complete lunar phase cycle, a “lunation”, is currently exactly 29 days, 12 hours, 44 minutes, and 3 seconds.

The Latin days of the week also reflect those planets closest to Earth—Mercury, Venus, Mars, Saturn, and Jupiter, plus the sun and moon.

Sunday is a working day in many Muslim countries and Israel. It is also a popular day in the United States and the United Kingdom to schedule televised sporting events.

Monday is considered a bad day because it is the first day of the working week, but in Judaism and Islam, it is considered a good day for fasting. It is also a day to commemorate angels in the Eastern Orthodox Church.

In the United States, Labor Day, Memorial Day, Columbus Day, and Veteran’s Day always fall on Mondays.

In Greek tradition, Tuesday is considered unlucky because Constantinople fell on a Tuesday. In Judaism, Tuesdays are lucky because in the Book of Genesis it is mentioned as a good day twice.

Wednesdays are days for fasting in the Eastern Orthodox Church.

In Australia, most shopping malls have “late night shopping” on Thursdays, as it is the day most Australians are paid.

Friday is considered an unlucky day to begin a voyage, but a good day for sowing the seed. Condemning a slave on Friday is forbidden under Muslim law.

Saturday is the Jewish day of rest, rather than Sunday as is common in most denominations of Christianity.

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