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?.
who invented the days of the week names
Roman Empire