<Ul> <Li> 1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year) </Li> <Li> 1 week = ⁄ ≈ 22.9984% of an average Gregorian month </Li> </Ul> <Li> 1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year) </Li> <Li> 1 week = ⁄ ≈ 22.9984% of an average Gregorian month </Li> <P> In a Gregorian mean year, there are 365.2425 days, and thus exactly 52 ⁄ or 52.1775 weeks (unlike the Julian year of 365.25 days or 52 ⁄ ≈ 52.1786 weeks, which cannot be represented by a finite decimal expansion). There are exactly 20,871 weeks in 400 Gregorian years, so 19 October 1617 was a Thursday just like 19 October 2017 . </P>

Who decided on 7 days in a week