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 nundinal cycle as the new religion spread. Sunday remained the first day of the week, being considered the Lord's Day, while the Jewish sabbath remained the seventh. Emperor Constantine adopted the seven-day week for official use in 321 CE, making the Day of the Sun () a legal holiday. In the international standard ISO 8601, Monday is treated as the first day of the week. Days named after planets Greco-Roman tradition Between the 1st and 3rd centuries CE, the Roman Empire gradually replaced the eight-day Roman nundinal cycle with the seven-day week. The earliest evidence for this new system is a Pompeiian graffito referring to 6 February (ante diem viii idus Februarias) of the year 60 CE as dies solis ("Sunday"). Another early witness is a reference to a lost treatise by Plutarch, written in about CE 100, which addressed the question of: "Why are the days named after the planets reckoned in a different order from the 'actual' order?". (The treatise is lost, but the answer to the question is known; see planetary hours). The Ptolemaic system of planetary spheres asserts that the order of the heavenly bodies, from the farthest to the closest to the Earth is: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, or, objectively, the planets are ordered from slowest to fastest moving as they appear in the night sky. The days were named after the planets of Hellenistic astrology, in the order: Sun (Helios), Moon (Selene), Mars (Ares), Mercury (Hermes), Jupiter (Zeus), Venus (Aphrodite) and Saturn (Cronos). The seven-day week spread throughout the Roman Empire in Late Antiquity. By the 4th century, it was in wide use throughout the Empire. The Greek and Latin names are as follows: Romance languages Except for Modern Portuguese and Mirandese, the Romance languages preserved the Latin names, except for the names of Sunday, which