The Christian, the Hebrew, and the Islamic calendars all have a 7-day week.
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 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.
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:
| English | Portuguese | Russian | Meaning of Russian name |
| Monday | segunda-feira | ponedelnik | After ``do-nothing'' |
| Tuesday | terça-feira | vtornik | Second |
| Wednesday | quarta-feira | sreda | Middle |
| Thursday | quinta-feira | chetverg | Fourth |
| Friday | sexta-feira | pyatnitsa | Fifth |
| Saturday | sabado | subbota | Sabbath |
| Sunday | domingo | voskresenye | Resurrection |
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:
| English | French | ``Planet'' |
| Monday | lundi | Moon |
| Tuesday | mardi | Mars |
| Wednesday | mercredi | Mercury |
| Thursday | jeudi | Jupiter |
| Friday | vendredi | Venus |
| Saturday | samedi | Saturn |
| Sunday | dimanche | (Sun) |
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.
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 BC), 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.
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''.
The Bible clearly makes Saturday (the Sabbath) the last day of the week. 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 mentioned in section 5.2). 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 has decreed that Monday shall be the first day of the week.
International standard ISO-8601 (mentioned in section 5.6) 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.
Note: This standard is not used in the United States.
If you know the date, how do you calculate the corresponding week number (as defined in ISO-8601)?
(I am very grateful to Stefan Potthast for this algorithm.)
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 section 6.1).
The Maya calendar uses a 13 and a 20-day ``week'' (see section 7.2).
The Soviet Union has 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.