The names of the days of the week

The names of the days of the seven-day week in many languages, including English, are derived from their being named after the classical planets in Hellenistic astrology, a system introduced in the Roman Empire during Late Antiquity. In other languages, the days are numbered, depending on regional tradition either beginning with Sunday or with Monday. In the international standard ISO 8601, Monday is treated as the first day of the week.