Why are there 7 days in a week?
The seven-day week comes from the ancient Near East. The Babylonians marked days tied to the Moon's phases, about 7 days apart, and saw seven moving lights in the sky: the Sun, the Moon and five planets. The Jewish week of six days and a Sabbath spread it through religion, and Rome made it official in 321 CE. Unlike the day, month and year, the week follows no single cycle in the sky.
A week that doesn't match the sky
A day is one turn of the Earth, a month roughly one cycle of the Moon, a year one orbit of the Sun. The week is different: it is a human invention. The Moon's cycle of about 29.5 days splits roughly into four quarters of 7.4 days, which is probably why seven felt natural, but it never fits exactly, and many cultures used other weeks. Ancient Egypt used 10-day weeks, Rome had an 8-day market cycle (the nundinae), and parts of West Africa used 4-day market weeks.
How seven days won
The Jewish week of six working days and a Sabbath rest, described in the Book of Genesis, spread with Judaism and then Christianity. Separately, astrologers in the Hellenistic world gave each hour and day a "ruling" planet, which produced a seven-day planetary week. The two merged in the Roman Empire, and in 321 CE Emperor Constantine made the Sun's day an official day of rest. Islam also uses the seven-day week, so it became almost universal.
Where the day names come from
| Day | Named after | Compare |
|---|---|---|
| Sunday | The Sun | Latin dies Solis |
| Monday | The Moon | French lundi (Luna) |
| Tuesday | Tiw (Týr), Norse god of war | French mardi (Mars) |
| Wednesday | Woden (Odin) | French mercredi (Mercury) |
| Thursday | Thor, god of thunder | French jeudi (Jupiter) |
| Friday | Frigg, wife of Odin | French vendredi (Venus) |
| Saturday | Saturn | Latin dies Saturni |
English swapped most of the Roman planet-gods for their closest Germanic equivalents, which is why Thor's day matches Jupiter's day. Portuguese went the other way and simply numbers its weekdays: segunda-feira (Monday) means "second day".
Attempts to change the week
Revolutionary France introduced a 10-day week, the décade, in 1793; workers got one rest day in ten instead of one in seven, and Napoleon abolished it in 1806. The Soviet Union tried a continuous 5-day week from 1929 and a 6-day week from 1931, before returning to seven days in 1940. Both failed largely because people wanted shared days off with family.
Why September is the ninth month
The earliest Roman calendar started the year in March and had ten months, so September (septem, seven), October (octo, eight), November (novem, nine) and December (decem, ten) really were the 7th to 10th months. January and February were added later, traditionally by King Numa, and from 153 BCE the year began on 1 January. Quintilis, the old fifth month, was renamed July for Julius Caesar in 44 BCE, and Sextilis became August for Emperor Augustus in 8 BCE.
Why February is shortest
February was the last month of the old Roman year, a time of purification rituals, and was left with the leftover days. When Julius Caesar reformed the calendar in 46 BCE, February kept 28 days and got the extra leap day every four years. The popular story that Augustus stole a day from February to make August as long as July is a medieval invention.
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Questions people ask
Why are there 7 days in a week?
Because the Babylonians, the Jewish Sabbath week and the Roman planetary week all used seven days, and Rome made it official in 321 CE. Seven also roughly matches a quarter of the Moon's cycle and the seven visible moving objects in the sky.
Who invented the 7-day week?
No single person. It grew from Babylonian moon-phase days, the Jewish Sabbath week and Greco-Roman astrology, and Emperor Constantine made it official in the Roman Empire in 321 CE.
Why is September the 9th month when "sept" means 7?
The early Roman year started in March, making September the 7th month. When the start of the year moved to January, the names stayed the same.
Has anyone tried a different week?
Yes. France used a 10-day week from 1793 to 1806, and the Soviet Union used 5- and 6-day weeks from 1929 to 1940.
Sources
- Eviatar Zerubavel, The Seven Day Circle: The History and Meaning of the Week, 1985.
- E. G. Richards, Mapping Time: The Calendar and Its History, Oxford University Press, 1998.
- Codex Justinianus 3.12.2: Constantine's law on the day of the Sun, 321 CE.
- Encyclopaedia Britannica, "Week" and "Roman republican calendar".