The following is an Encore presentation of Everything Everywhere Daily.

Most of our major divisions of time are based on some natural event. A year is one orbit of the Earth around the sun. A month is one orbit of the moon around the Earth, and a day is a rotation of the Earth about its axis. However, one of the most commonly used units of time has no natural analog whatsoever. Learn more about why there are seven days in a week and where the names for the days the week come from on this episode of Everything Everywhere Daily. I've done many episodes on how our calendar came to be. I've talked about how the Gregorian calendar fixed the Julian calendar. I've done episodes on why January first is the beginning of the year. I've talked about the Roman origin of where our months come from, and I've also got into way too much detail talking about why we started numbering years the way we do. So I have a thing about time and calendars. Nonetheless, despite all the episodes I've done on this subject, there's one thing I haven't touched yet, the week. A week has seven days, yet there's nothing in nature that's on a seven-day cycle.

Unlike years, months, and days, a week is an entirely arbitrary unit of time. So why are there seven days in a week? Not five or nine or something else. I should note that there were other ancient civilizations that did have a different number of days in their week. The Egyptians had a 10-day week. The Atruscans had an eight-day week, which was adopted by the early Romans. Likewise, the Celts had an eight-day week, but a Keltic fortnight, which is where the term originates, was actually 15 days. One of the problems with making a calendar is that none of the natural units of time are multiples of each other. So no matter what you try to do, there will be a fractional number of days in a month or a year. The average length of a lunar month, the time it takes for one full cycle of a phase of the moon, is approximately 29 and a half days, which can vary by as much as four hours throughout the year. A week is just a way to find a middle ground between a day and a much longer month. The Egyptian idea of a 10-day week actually fits nicely into a period of almost 30 days.

Three Egyptian weeks would make up one month. But you could also divide that into five or six day weeks as well. Seven divides evenly into 28, which is also close to 29.5, but isn't as accurate as 10, 6, or 5. So why seven? The people who gave us the seven day week were the Babylonians. The Babylonians associated the number seven with the seven planets. As I've mentioned before, the ancient definition of a planet was anything that moved in the sky. So the seven planets to them would have been the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Babylonians associated each day of their seven-day week with one of the planets, and this was developed almost 4,000 years ago. The Babylonians also had the equivalent of a leap day every month to make the weeks line up with the moon. So why did the Babylonian seven-day week win out over the 10 or 8-day week? The Babylonians were uniquely situated between empires in the East and the West. To the East, the Persians adopted the Babylonian tradition of using a seven-day week. To the West, the Jews and the Kingdom of Judea used a seven-day week to celebrate their Sabbath.

The Book of Genesis also had God creating the world in seven days, which was the original basis for them of the week. The Jewish Babylonian captivity cemented the seven-day week for the Jews for everything, not just religious calendars. To the West, again, the Greeks adopted the seven-day week sometime around the year 400 BC. When Alexander the Great conquered his way across Asia, he brought the seven-day week with him and introduced to India. The Tang dynasty began using a seven-day week in the seventh century and were probably introduced via India or Southeast Asia. However, there were some early mentions of a seven-day week as early as the fourth century Jin dynasty. The Romans used the seven-day week alongside their eight-day week, but eventually the seven-day week was made official by Emperor Constantine in 321. He did this because it was already in widespread use and conformed to the Christian calendar, which adopted Sunday as their Holy Day. For good measure, Constantine made Sunday a holiday for everyone, pagans included, who just called it the Sun's Day. The early Christians called it the Lord's Day. The Islamic calendar also has a seven-day week that is based on the story of creation.

The major difference between the Islamic story of creation and the account in Genesis is that God started creation on Saturday and ended by creating humans just before the evening on Friday. The association of the Days of the week with ancient planets is one that has remained since the Babylonians. The order of the days named after the planets was Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. This particular ordering was a Greek and Roman tradition, which is still handed down to us today. In many romance languages, the days of the week correspond much closer to the planet's names than in English or Germanic languages. For example, Monday in Spanish is lunes, and in French, it's Lundi, which comes from luna, the Latin word for moon. Tuesday is Martes in Spanish and mardi in French, which come from Mars. Wednesday is mercoles in Spanish and mercredi in French, which come from Mercury. Thursday is jueves in Spanish, spelled with a J, and judi in French, both named after Jove, another name for Jupiter. Friday is Viernes in Spanish, and Vendredi in French, which come from Venus. Saturday is Sabado in Spanish, and Samedi in French, which come from Saturn.

The only odd one is Sunday, which is Domingo in Spanish and dimanche in French. Both of these words come from the Latin word for the Lord's Day, Dominicus. So the romance languages seem to have stuck to the original meanings for the planets. For English and Germanic languages, however, words for the days of the week don't seem to have much to do with the planets. This has to do with the roots of English coming from the Germanic Saxons who invaded England and brought their language with them. Even though about half the vocabulary in English has Latin roots, the days of the week are not among them. Sunday and Monday are pretty obvious. They are named after the sun and the moon. In Old English, they were known as sun and day and monunday, or Sunday and moon day. The other day which does reference a planet is Saturday, which in Old English was Saturnis die, or saturn's day. The other four days don't seem to have any planetary reference at all. So why is that? In the Roman the planets were named after Roman gods, so every planet's name is also the name of a god.

The ancient Germans swapped out the names of Roman gods for the equivalent Norse gods. Mars was the Roman God of war. The Norse god of combat was Tyr, which was also known as Tew, T-I-W. So the day of Mars became the day of Tew, which was known as Tew's Die in Old English. Mercury was swapped out for Oden. Oden was considered to be the equivalent Mercury, not because Oden was a messenger God, but because Oden also had the role of escorting souls to the afterlife. The day of Mercury became the day of Oden, which in Old English was pronounced Woden. Hence, the day of the week was now known as Woden's Die. Jupiter or Jove was exchanged for Thor. Jupiter was the God of Thunder and Lightning, as was Thor. So in old English, it was pronounced Thurren's Die, which was then morphed into Thursday. For Friday, Venus was replaced with Frigg or Frigga, the wife of Oden. This replacement was simply because Frigga was one of the most significant goddesses in the Norse pantheon. In Old English, it was Friggidae, which became Friday. So to summarize, the origins of the days of the week in English: sun, moon, tire, Oden, Thor, Frigga, Saturn.

At this point, I haven't mentioned one of the most important parts of any week that you probably consider all the time: the work week and the weekend. The concept of a weekend and a work week is actually a rather modern invention. It didn't really take hold until the early 20th century. But in Jewish and Christian traditions, there had always been at least one day of the week dedicated to rest, Saturday for Jews and Sundays for Christians. However, the idea of taking both days together as time off from work is only a little over 100 years old. Every country on Earth has at least one day off every week, with most countries having two days off. The days which are taken off will differ in most countries. In many Muslim countries, Friday is a part of the weekend or is the only day off. Fridays in Arabic is known as Al-Jumuhah, which means the day of a congregation. It is religious, but it's not the equivalent of the Christian a Jewish Sabbath, as it's not considered a day of rest. While Muslims are required to pray every day, Fridays are the day men are encouraged to gather together at a mosque.

The fact that three different days have religious significance for three different religions means that countries have different definitions of a weekend. Saturday and Sunday are the most popular days off all over the world. Much of the Islamic world and Israel actually take off Fridays and Saturdays. However, some Muslim countries, such as Morocco, Malaysia, and Indonesia, have a a Sunday weekend with an extended midday break on Fridays to attend mosque. The nation of Brunei, as well as some provinces of Indonesia and Malaysia, have off Fridays and Sundays with a workday in between on Saturday. With there not being any universal days for a work week or a weekend, there's also disagreement as to what is the first day of the week. The world is broadly split on the subject. About 55% of the world consider Sunday to be the first day of the week, and 45% consider Monday to be the first day of the week. Iran, Somalia, and Afghanistan usually have their week starting on Saturday. In the United States, most calendars will list Sunday as the first day of the week, but most people treat Monday as the first day of the week as it's the start of the work week.

The concept of a week is a handy thing, but it's broadly lacking in any international standards. But so long as everyone is on the same page in terms of days, months, and years, what week something is really doesn't matter that much. So if you look at our calendar, especially if you speak English, we have a very odd system. Our numbering system for years is Christian, and the names of the month are Roman, and the days of the week are Norse. The entire seven-day week system is Babylonian. The divisions of a day into 24 hours comes from ancient Egypt, and the division of hours into 60 minutes and minutes into 60 seconds was developed by Islamic scholars who were using a Bayes' 60 numbering system developed by ancient Samarians. It is a system that absolutely nobody would have designed from scratch, yet it somehow works quite well.

The executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Benjie Long and Cameron Kiever. I want to give a big shout out to everyone who supports the show over on Patreon, including the show's producers. Your support helps me put out a show every single day. And also, Patreon is currently the only place where Everything Everywhere Daily merchandise is available to the top tier of supporters. If you'd like to talk to other listeners of the show and members of the Completionist Club, you can join the Everything Everywhere Daily Facebook group or Discord server. Links to everything are in the show notes.