Sabbath a synonym of midnight-to-midnight "Saturday" (literally, Saturn's day in at least a dozen languages): this is a simplification of the use of "Sabbath" in other religious contexts, where the two do not coincide. (Using midnight instead of sundown as delimiter dates back to the Roman Empire.) In over thirty other languages, the common name for this day in the seven-day week is a cognate of "Sabbath". "Sabbatini", originally "Sabbadini", often "Sabatini", etc., is a very frequent Italian name form ("Sabbatos" is the Greek form), indicating a family whose ancestor was born on Saturday, Italian "sabato"; "Domenico" indicated birth on Sunday.
Eight-day week popularity of the seven-day rhythm won and the eight-day week disappeared. The cycle of seven days, named after the sun, the moon, and the five planets visible to the naked eye, was already customary in the time of Justin Martyr, who wrote of the Christians meeting on the Day of the Sun (Sunday). Emperor Constantine eventually established the seven-day week in the Roman calendar in AD 321. The Celts 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