Some historical calendars had "

week A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are ofte ...

s" consisting of eight days.

Burma

In the Burmese version of Theravada Buddhism, the week has eight days. Wednesday is divided into Wednesday proper (midnight to noon) and Rahu (noon to midnight). Each day is associated with a compass direction, a planet, and a totem animal.

Nundinum

The ancient

Etruscans The Etruscan civilization ( ) was an ancient civilization created by the Etruscans, a people who inhabited Etruria in List of ancient peoples of Italy, ancient Italy, with a common language and culture, and formed a federation of city-states. Af ...

developed an eight-day market week known as the nundinum around the 8th or 7th century BC. This was passed on to the Romans no later than the 6th century BC. As Rome expanded, it encountered the seven-day week and for a time attempted to include both. The popularity of the seven-day rhythm won, and the eight-day week disappeared. The cycle of seven days – named for the sun, the moon, and the five planets visible to the naked eye – was already customary in the time of

Justin Martyr Justin, known posthumously as Justin Martyr (; ), also known as Justin the Philosopher, was an early Christian apologist and Philosophy, philosopher. Most of his works are lost, but two apologies and a dialogue did survive. The ''First Apolog ...

, who wrote of the Christians meeting on the Day of the Sun (Sunday). Emperor

Constantine Constantine most often refers to: * Constantine the Great, Roman emperor from 306 to 337, also known as Constantine I * Constantine, Algeria, a city in Algeria Constantine may also refer to: People * Constantine (name), a masculine g ...

eventually established the seven-day week in the

Roman calendar The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. Although the term is primarily used for Rome's pre-Julian calendars, it is often used inclusively of the Julian calendar established by Julius Caesar in 46&nbs ...

in AD 321.

Celtic calendar

The

Celts The Celts ( , see Names of the Celts#Pronunciation, pronunciation for different usages) or Celtic peoples ( ) were a collection of Indo-European languages, Indo-European peoples. "The Celts, an ancient Indo-European people, reached the apoge ...

used periods of darkness such as night and winter to begin their calculations of time. This meant that the first period of time in a "week" was a night, followed by a day. Further, they also counted the ending night period, giving rise to periods of time with more nights than days. In Old Irish, the term ''nómad'' is used to signify a number of days. The usage of the term varies and there are different theories about the length of time involved, but they all involve nine periods of some kind, e.g. nine days & nights; 9 × 8 hours = 72 hours = 3 days & nights; 9 × 9 hours ~ 3½ days; 9 × 12 hours = 4½ days. Joseph Loth noted the frequency of the expression ''co cend nomaide'' (“to the end of a ''nómad''”) in the literature—sometimes replaced by ''nóilaíthe'' (“nine days”)—and how the division fitted neatly into a

sidereal month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...

of 27 nights. In Welsh, the word for "week" is ''wythnos'' which literally means "eight-nights" since it was historically considered that a week started and ended with a period of night bracketing seven days. Similarly the word for "fortnight", ''pythefnos'', literally means "fifteen-nights". The Irish word for "fortnight", '' coicís'', also relates to the number fifteen.

References

{{Time measurement and standards Weeks