<P> ISO 8601 includes the ISO week date system, a numbering system for weeks within a given year--each week begins on a Monday and is associated with the year that contains that week's Thursday (so that if a year starts in a long weekend Friday--Sunday, week number one of the year will start after that). ISO 8601 assigns numbers to the days of the week, running from 1 to 7 for Monday through to Sunday . </P> <P> The term "week" is sometimes expanded to refer to other time units comprising a few days, such as the nundinal cycle of the ancient Roman calendar or the "work week" or "school week" referring only to the days spent on those activities . </P> <P> The English word week comes from the Old English wice, ultimately from a Common Germanic * wikōn -, from a root * wik - "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the Roman calendar, perhaps "succession series", as suggested by Gothic wikō translating taxis "order" in Luke 1: 8 . </P> <P> The seven - day week is named in many languages by a word derived from "seven". The archaism sennight ("seven - night") preserves the old Germanic practice of reckoning time by nights, as in the more common fortnight ("fourteen - night"). Hebdomad and hebdomadal week both derive from the Greek hebdomás (ἑβδομάς, "a seven"). The obsolete septimane is cognate with the Romance terms derived from Latin septimana ("a seven"). </P>

How did we get 7 days in a week