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Snippet from Wikipedia: Names of the days of the week

In a vast number of languages, the names given to the seven days of the week (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday) are derived from the names of the seven heavenly bodies (the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn) which were in turn named after contemporary Hellenistic deities. This system was introduced by the Babylonians and later adopted by the Sumerians. The Roman Empire adopted the system during late antiquity. In some other languages, the days are named after corresponding deities of the regional culture. The seven-day week was adopted in early Christianity from the Hebrew calendar, and gradually replaced the Roman internundinum. Eight-day and seven-day weeks existed side-by-side until Emperor Constantine made the seven-day week official in AD 321; thereafter, the seven-day week spread throughout the Roman Empire and eventually through Christian cultures around the world.

The history of the seven-day week can be traced to ancient civilizations. 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. Most historians agree the seven-day week dates back to Babylonians who started using it about 4,000 years ago. The number 7 was sacred to the Babylonians. Emperor Constantine of the Roman Empire made the Day of the Sun (dies Solis, "Sunday") a legal holiday centuries later.

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.

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Snippet from Wikipedia: Day

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, 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 exist for marking the beginning and end of days, such as the Jewish religious calendar which counts days beginning at sunset, or in astronomy, in which 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 some 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). It is a non-SI unit that is accepted for use with SI.

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