when did we start using the days of the week
Week The seven-day week seems to have been adopted, at different stages, by the Persian Empire, in Hellenistic astrology, and (via Greek transmission) in Gupta India and Tang China.[b][citation needed]
Names of the 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 by 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.