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]
Sunday The international standard ISO 8601 for representation of dates and times, states that Sunday is the seventh and last day of the week.[5] This method of representing dates and times unambiguously was first published in 1988.