Days of the week

The names of the days of the week in many languages are derived from the names of the classical planets in Hellenistic astrology, which were in turn named after contemporary deities, a system introduced in the Roman Empire during Late Antiquity. In some other languages, the days are named after corresponding deities of the regional culture, either beginning with Sunday or with Monday. In the international standard ISO 8601, Monday is treated as the first day of the week.

Between the 1st and 3rd centuries the Roman Empire gradually replaced the eight-day Roman nundinal cycle with the seven-day week. Our earliest evidence for this new system is a Pompeiian graffito referring to the 6th February (viii idus Februarius) of the year AD 60 as dies solis ("Sunday"). Another early witness is a reference to a lost treatise by Plutarch, written in about AD 100, which addressed the question of Why are the days named after the planets reckoned in a different order from the actual order?.

The days were named after the planets of Hellenistic astrology, in the order Sun, Moon, 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, and it had also reached India and .

The Greek and Latin names are as follows:

Except for modern Portuguese, the Romance languages preserved the Latin names, except for the names of Sunday, which was replaced by [dies] Dominicus (Dominica), i.e. "Day of the Lord" and of Saturday, which was named for the Sabbath.

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