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 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").[1] 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?.[2]