<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 2 January 1618 was a Tuesday just like 2 January 2018 . </P> <P> Relative to the path of the Moon, a week is 23.659% of an average lunation, or 94.637% of an average quarter lunation . </P> <P> Historically, the system of Dominical letters (letters A to G identifying the weekday of the first day of a given year) has been used to facilitate calculation of the day of week . The day of the week can be easily calculated given a date's Julian day number (JD, i.e. the integer value at noon UT): Adding one to the remainder after dividing the Julian day number by seven (JD modulo 7 + 1) yields that date's ISO 8601 day of the week (for example, the Julian day number of 2 January 2018 is 2458121 . Calculating (2458121 mod 7 + 1) yields 2, corresponding to Tuesday .). </P> <P> The days of the week were originally named for the classical planets . This naming system persisted alongside an "ecclesiastical" tradition of numbering the days, in ecclesiastical Latin beginning with dominica (the Day of the Lord) as the first day . The Greco - Roman gods associated with the classical planets were rendered in their interpretatio germanica at some point during the late Roman Empire, yielding the Germanic tradition of names based on indigenous deities . </P>

Where did the seven day week come from