Our seven day week has been used for millennia by the Christian, Jewish, Islamic, and Chinese calendars, yet its origins are most uncertain.

What Is the Origin of the 7-Day Week?
What Do the Names of the Days of the Week Mean?
What is the System behind the Planetary Day Names?
Has the 7-Day Week Cycle Ever Been Interrupted?
What Is the First Day of the Week?
What Is the Week Number?
How can I calculate the week number?
Do Weeks of Different Lengths Exist?
What day was a certain date?

Digging into the history of the 7-day week is a very complicated matter. Authorities have very different opinions about the history of the week, and they frequently present their speculations as if they were indisputable facts. The only thing we seem to know for certain about the origin of the 7-day week is that we know nothing for certain.

The common explanation is that the seven-day week was established as imperial calendar in the late Roman empire and furthered by the Christian church for historical reasons. The British Empire used the seven-day week and spread it worldwide. Today the seven-day week is enforced by global business and media schedules, especially television and banking.The first pages of the Bible explain how God created the world in six days and rested on the seventh. This seventh day became the Jewish day of rest, the sabbath, Saturday.Extra-biblical locations sometimes mentioned as the birthplace of the 7-day week include: Babylon, Persia, and several others. The week was known in Rome before the advent of Christianity.

There are practical geometrical theories as well. For example, if you wrap a rubber band around 7 soda cans (or any other convenient circular objects). You get a perfect hexagon with the 7th can in the middle. It is the only stable configuration of wrapping more than 3 circular objects. Four, 5, and 6 objects will slip from one configuration to another. Ancients wrapping tent poles, small logs for firewood, or other ciruclar objects might have come upon this number and attach a mystical significance to it.

One viable theory correlates the seven day week to the seven (astrological) "planets" known to the ancients: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The number seven does not seem an obvious choice to match lunar or solar periods, however. A solar year could be more evenly divided into weeks of 5 days, and the moon phases five-day and six-day weeks make a better short term fit (6 times 5 is 30) to the lunar (synodic) month (of about 29.53 days) than the current week (4 times 7 is 28). The seven-day week may have been chosen because its length approximates one moon phase (one quarter = 29.53 / 4 = 7.3825).

What Do the Names of the Days of the Week Mean?

An answer to this question is necessarily closely linked to the language in question. Whereas most languages use the same names for the months (with a few Slavonic languages as notable exceptions), there is great variety in names that various languages use for the days of the week. A few examples will be given here.

Except for the sabbath, Jews simply number their week days.

A related method is partially used in Portuguese and Russian:

Most Latin-based languages connect each day of the week with one of the seven "planets" of the ancient times: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. French, for example, uses:

The link with the sun has been broken in French, but Sunday was called dies solis (day of the sun) in Latin.
It is interesting to note that also some Asiatic languages (for example, Hindi, Japanese, and Korean) have a similar relationship between the week days and the planets.English has retained the original planets in the names for Saturday, Sunday, and Monday. For the four other days, however, the names of Anglo-Saxon or Nordic gods have replaced the Roman gods that gave name to the planets. Thus, Tuesday is named after Tiw, Wednesday is named after Woden, Thursday is named after Thor, and Friday is named after Freya.

See additional connotations of the days of the week from a sampling of cultures and time periods.

As we saw in the previous section, the planets have given the week days their names following this order:

Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun

Why this particular order?

One theory goes as follows: If you order the "planets" according to either their presumed distance from Earth (assuming the Earth to be the center of the universe) or their period of revolution around the Earth, you arrive at this order: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn

Now, assign (in reverse order) these planets to the hours of the day:

1=Saturn, 2=Jupiter, 3=Mars, 4=Sun, 5=Venus, 6=Mercury, 7=Moon, 8=Saturn, 9=Jupiter, etc., 23=Jupiter, 24=Mars

Then next day will then continue where the old day left off:

1=Sun, 2=Venus, etc., 23=Venus, 24=Mercury

And the next day will go

1=Moon, 2=Saturn, etc.

If you look at the planet assigned to the first hour of each day, you will note that the planets come in this order:

Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus   

This is exactly the order of the associated week days.    

Coincidence? Maybe.

There is no record of the 7-day week cycle ever having been broken. Calendar changes and reform have never interrupted the 7-day cycles. It very likely that the week cycles have run uninterrupted at least since the days of Moses (c. 1400 B.C.E.), possibly even longer.

Some sources claim that the ancient Jews used a calendar in which an extra Sabbath was occasionally introduced. But this is probably not true.

Which Day is the Day of Rest?

For the Jews, the Sabbath (Saturday) is the day of rest and worship. On this day God rested after creating the world.

Most Christians have made Sunday their day of rest and worship, because Jesus rose from the dead on a Sunday.

Muslims use Friday as their day of rest and worship. The Qur'an calls Friday a holy day, the "king of days."

What Is the First Day of the Week?

The Bible clearly makes the Sabbath the last day of the week, but does not share how that corresponds to our 7 day week. Yet through extra-biblical sources it is possible to determine that the Sabbath at the time of Christ corresponds to our current 'Saturday.' Therefore it is common Jewish and Christian practice to regard Sunday as the first day of the week (as is also evident from the Portuguese names for the week days). However, the fact that, for example, Russian uses the name "second" for Tuesday, indicates that some nations regard Monday as the first day.

In international standard ISO-8601 the International Organization for Standardization (ISO) has decreed that Monday shall be the first day of the week.

What Is the Week Number?

International standard ISO-8601 assigns a number to each week of the year. A week that lies partly in one year and partly in another is assigned a number in the year in which most of its days lie. This means that

Week 1 of any year is the week that contains 4 January,
or equivalently
Week 1 of any year is the week that contains the first Thursday in January.

Most years have 52 weeks, but years that start on a Thursday and leap years that start on a Wednesday have 53 weeks.

If you know the date, how do you calculate the corresponding week number (as defined in ISO-8601)?

1. Using the formulas in the section on the Christian calendar, calculate the Julian Day Number, J.

2. Perform the following calculations (in which the divisions are integer divisions in which the remainder is discarded):

d4 = (J+31741 - (J mod 7)) mod 146097 mod 36524 mod 1461
L = d4/1460
d1 = ((d4-L) mod 365) + L
WeekNumber = d1/7+1

If you define a "week" as a 7-day period, obviously the answer is no. But if you define a "week" as a named interval that is greater than a day and smaller than a month, the answer is yes.

The ancient Egyptians used a 10-day "week", as did the French Revolutionary calendar (see French calendar).

The Maya calendar uses a 13 and a 20-day "week" (see Mayan calendar).

The Soviet Union used both a 5-day and a 6-day week. In 1929-30 the USSR gradually introduced a 5-day week. Every worker had one day off every week, but there was no fixed day of rest. On 1 September 1931 this was replaced by a 6-day week with a fixed day of rest, falling on the 6th, 12th, 18th, 24th, and 30th day of each month (1 March was used instead of the 30th day of February, and the last day of months with 31 days was considered an extra working day outside the normal 6-day week cycle). A return to the normal 7-day week was decreed on 26 June 1940.

Lithuanians used week of nine days before adopting Cristianity.

What day was a certain date?

To calculate the day on which a particular date falls, the following algorithm may be used (the divisions are integer divisions, in which remainders are discarded). In July 4, 1950, date=4, month=7, year=1950. Note that "mod" means the remainder when doing integer division, e.g., 20 mod 7 = 6. That is, 20 divided by 7 is 2 and 6/7th (where xsi is the remainder).

Where day 0 is Sunday, day 1 is Monday, etc.

The week of seven days was adopted in Rome somewhere about 400 AD, and spread into Europe, but had been recognized as a period of time long before that in the east. It was probably chosen to give one day each to each of the seven planets known in antiquity. In the southern countries of Europe, the days of the week were named after the gods of the Greeks and Romans. In the English language, as well as in the languages of some of the countries of northern Europe, the gods of the north have given their names to the days.

The Ashanti and some other peoples of West Africa gave a child a special name according to the day of the week on which he was born. The habit was brought to the American South and Caribbean through slavery, where names such as Quashee (Sunday), Cudjo (Monday) and Cufee (Friday) were common.

Monday
Monday is the second day of the week, day of moon goddess, Selene, Luna and Mani.
Derived from Lunae Dies, day of the moon, the name reflects the ancient observance of feast days dedicated to moon goddess or planet.

The metal silver, dedicated to the moon, is associated with Monday.

Tuesday
Tuesday is the third day of the week. In the Roman calendar the corresponding day was dies Martis, the day of Mars, associated with Ares. Tiw's day is derived from Tyr or Tir, the god of honorable war, the wrestler and the son of Odin and, or Woden, the Norse god of war and Frigga, the earth mother. His emblem is the sword, and in olden days the people paid him great homage. Tuesday was named in his honor.

The metal iron, dedicated to Mars and interpreted as his spear and shield, is an attribute of Tuesday

Wednesday
Wednesday, the fourth day of the week, corresponds to the Roman Dies Mercurii. The name derives from the Scandinavian Woden (Odin), chief god of Norse mythology, who was often called the All Father.
Quicksilver, a liquid mercury that contains amounts of the platinum group metals, has been interpreted as the caduceus of the Greek Hermes (Mercury in Roman myth), and is therefore an attribute of Wednesday

Thursday
Thursday is the fifth day of the week. It derives its name from the Middle English Thoresday, or Thursdaye, corresponding to the Roman dies Jovis.

Thor, the god of strength and thunder, defender and help in war, son of Odin, is the counterpart of Jupiter or Jove. Thor is one of the twelve great gods of northern mythology. He is the only god who cannot cross from earth to heaven upon the rainbow, for he is so heavy and powerful that the gods fear it will break under his weight. It was said that whenever Thor threw his hammer, the noise of thunder is heard through the heavens. Thursday was sacred to Thor.

The metal tin is associated with the thunderbolt of Jupiter (Zeus in Greek myth) and is an attribute of Thursday.

I am the Thunderer!
Here in my Northland,
My fastness and fortress,
Reign I forever!
--Henry Wadsworth Longfellow

Friday

Friday is the sixth day of the week. The name is derived from the Germanic Frigga the name of the Norse god Odin's wife. Frigga is considered to be the mother of all, and the goddess who presides over marriage. The name means loving or beloved.

The corresponding Latin name is Dies Veneris, a day dedicated to Venus, the goddess of love.

The metal copper, dedicated to Venus, is associated with Friday.

Saturday
Saturday is the seventh day of the week, corresponding to the Roman dies Saturni, or day of Saturn, the Roman god of agriculture.

Saturday is also represented by Loki, the Norse god of tricks and chaos.

The metal lead is associated with the scythe of Saturn, and is therefore an attribute of Saturday.

Sunday
Sunday is the first day of the week.

From prehistoric times to the close of the fifth century of the Christian era, the worship of the sun was dominant.

Sunday celebrates the sun god, Ra, Helios, Apollo, Ogmios, Mithrias, the sun goddess, Phoebe.

The metal gold, as dedicated in the symbols of alchemy, is associated with the sun god and Sunday.

In the year 321, Constantine the Great ruled that the first day of the week, 'the venerable day of the sun', should be a day of rest. The sun's old association with the first day is responsible for the fact that the Lord's Day of Christianity bears the pagan name of Sunday.

The names of the days are in some cases derived from Teutonic deities or, such as in Romance languages, from Roman deities. The early Romans, around the first century, used Saturday as the first day of the week. As the worshipping of the Sun increased, the Sun's day (Sunday) advanced from position of the second day to the first day of the week (and saturday became the seventh day).