See Names of the days of the week and days of the week
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In many languages, the names given to the seven days of the week are derived from the names of the classical planets in Hellenistic astronomy, which were in turn named after contemporary deities, a system introduced by the Sumerians and later adopted by the Babylonians from whom the Roman Empire adopted the system during late antiquity. In some other languages, the days are named after corresponding deities of the regional culture, beginning either with Sunday or with Monday. The seven-day week was adopted in early Christianity from the Hebrew calendar, and gradually replaced the Roman internundinum.
Sunday remained the first day of the week, being considered the day of the sun god Sol Invictus and the Lord's Day, while the Jewish Sabbath remained the seventh. Emperor Constantine adopted the seven-day week for official use in 321 AD, making the Day of the Sun (dies Solis, "Sunday") a legal holiday.
In the international standard ISO 8601, Monday is treated as the first day of the week, but in many countries it is counted as the second day of the week.
Further reading Germanic calendar
The Germanic peoples adapted the system introduced by the Romans by substituting the Germanic deities for the Roman ones (with the exception of Saturday) in a process known as interpretatio germanica.
The date of the introduction of this system is not known exactly, but it must have happened later than CE 200 but before the introduction of Christianity during the 6th to 7th centuries, i.e., during the final phase or soon after the collapse of the Western Roman Empire. This period is later than the Common Germanic stage, but still during the phase of undifferentiated West Germanic. The names of the days of the week in North Germanic languages were not calqued from Latin directly, but taken from the West Germanic names.
(
), meaning “sun's day”. This is a translation of the Latin phrase
. English, like most of the Germanic languages, preserves the day's association with the sun. Many other European languages, including all of the Romance languages, have changed its name to the equivalent of “the Lord's day” (based on Ecclesiastical Latin
). In both West Germanic and North Germanic mythology, the Sun is personified as Sunna/Sól.
(
), meaning “Tiw's day”. Tiw (Norse
) was a one-handed god associated with single combat and pledges in Norse mythology and also attested prominently in wider Germanic paganism. The name of the day is also related to the Latin name
, “Day of Mars” (the Roman god of war).
(
) meaning the day of the Germanic god Woden (known as Óðinn among the North Germanic peoples), and a prominent god of the Anglo-Saxons (and other Germanic peoples) in England until about the seventh century. This corresponds to the Latin counterpart
, “Day of Mercury”, as both are deities of magic and knowledge. The German Mittwoch, the Low German
, the miðviku- in Icelandic
and the Finnish
all mean “mid-week”.
(
). In Latin, it was
, “Day of Saturn”. The Nordic laugardagur, leygardagur, laurdag, etc. deviate significantly as they have no reference to either the Norse or the Roman pantheon; they derive from Old Nordic
, literally “washing-day”. The German
(mainly used in northern and eastern Germany) and the Low German
mean “Sunday Eve”; the German word
derives from the name for Shabbat.
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.
A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon's lunar phase.
In common usage, a day starts at midnight, written as 00:00 or 12:00 am in 24- or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time. Other conventions are sometimes used, for example the Jewish religious calendar counts days from sunset to sunset, so the Jewish Sabbath begins at sundown on Friday. In astronomy, a day begins at noon so that observations throughout a single night are recorded as happening on the same day.
In specific applications, the definition of a day is slightly modified, such as in the SI day (exactly 86,400 seconds) used for computers and standards keeping, local mean time accounting of the Earth's natural fluctuation of a solar day, and stellar day and sidereal day (using the celestial sphere) used for astronomy. In most countries outside of the tropics, daylight saving time is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day. Due to slight variations in the rotation of the Earth, there are rare times when a leap second will get inserted at the end of a UTC day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been a total of 27 leap seconds that have been inserted, so roughly once every other year).