History of days names
Days of the week in many world languages are derived from the names of the planets in Hellenistic astrology. In their turn they were named after deities. Such system was introduced by the Roman Empire during Late Antiquity. In some languages, the days are named after corresponding deities of the regional culture, where week may begin with Sunday or with Monday. Monday is consdered as the first day of the week according to the international standard ISO 8601.
Planets-related names of days
Through 1st – 3rd centuries, the Roman Empire gradually replaced the eight-day week with the seven-day week. The earliest evidence for this replaced system is a Pompeiian graffito dated back to 6 February of AD 60 as dies solis (“Sunday”).
The days were named after the planets of Hellenistic astrology, in the following 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 widely used throughout the Empire, and even reached India and China.
Days in Romance languages
Apart from modern Portuguese and Mirandese, the Romance languages preserved Latin names, excluding Sunday, which was replaced by [dies] Dominicus (Dominica), i.e. “the Lord’s Day” and Saturday, named for the Sabbath. In Corsican, the Saturday is also known as Sabatu or De Sadorn.
Days in Celtic languages
Early Old Irish adopted the names from Latin, but introduced separate terms of Norse origin for Wednesday, Thursday and Friday, later on superseded them with terms of church fasting practices.