Where the seven-day week came from
The week is the only common unit of time that is not tied directly to the sun or the moon. A lunar month lasts about 29.5 days, and dividing it into four parts gives roughly seven days, which may be one reason the number took hold.
Ancient Mesopotamian cultures gave special importance to the seventh day and to the seven visible wandering bodies of the sky. The Jewish week, ending with the Sabbath, made a strict seven-day cycle part of religious life. In the Roman world, a seven-day week named after the planets spread during the early centuries AD, and in 321 AD the emperor Constantine made Sunday an official day of rest.
The planetary order
The Romans named each day after one of the seven classical planets, which included the Sun and the Moon. The order of the days comes from an old astrological system:
- Ancient astronomers ranked the seven bodies by how slowly they appear to move: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon.
- Each hour of the day was assigned to a planet in that order, repeating.
- Each day was named after the planet that ruled its first hour.
Because 24 hours is three more than three full rounds of seven, the ruling planet of the first hour shifts three places each day. Starting with Saturn’s day, the next first hour falls to the Sun, then the Moon, then Mars, Mercury, Jupiter and Venus. That gives the order still used today: Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday.
The English names
When Germanic peoples adopted the Roman week, they kept the Sun and Moon but swapped most Roman gods for similar figures from their own mythology:
| Day | Old English source | Named after | Roman equivalent |
|---|---|---|---|
| Sunday | Sunnandæg | The Sun | Sol |
| Monday | Mōnandæg | The Moon | Luna |
| Tuesday | Tīwesdæg | Tiw, a god of war | Mars |
| Wednesday | Wōdnesdæg | Woden (Odin) | Mercury |
| Thursday | Þunresdæg | Thunor (Thor), god of thunder | Jupiter |
| Friday | Frīgedæg | Frigg, wife of Woden | Venus |
| Saturday | Sæternesdæg | Saturn | Saturn |
The matches follow rough similarities: Tiw and Mars were both war gods, Thor and Jupiter both ruled thunder, and Frigg, like Venus, was linked with love and marriage.
Other languages
Romance languages kept the Roman names more directly. French has lundi (Moon), mardi (Mars), mercredi (Mercury), jeudi (Jupiter) and vendredi (Venus). Spanish has lunes, martes, miércoles, jueves and viernes. Their weekend names changed under Christian influence: Sunday became the Lord’s day (dimanche, domingo), and Saturday in Spanish became sábado, from Sabbath.
The planetary week traveled east as well. In Hindi, the days are named after the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn in the same order. In Japanese, the days are named after the Sun, the Moon and the five classical elements linked with the planets: fire, water, wood, metal and earth.
Some languages count the days instead. Portuguese uses segunda-feira (second day) for Monday through sexta-feira (sixth day) for Friday, and in modern Greek, Monday through Thursday are named second through fifth.
Which day starts the week?
Religious traditions treat Sunday as the first day, and many calendars in the United States still start on Sunday. The international standard ISO 8601 makes Monday the first day and numbers weeks from Monday to Sunday. Our week number tool follows the ISO system, and the printable calendar lets you choose which day starts the week.
Finding the weekday of any date
Because the week cycles without interruption, any date can be matched to its weekday. The weekday calculator does it instantly, and the doomsday calculator shows a mental method for working it out by hand. For the story behind the other half of a date, read about the origin of month names.