Editing Week (section)

==Definition and duration==
A week is defined as an interval of exactly seven [[day]]s,{{efn|1=In pre-modern times, days were measured either from sunset to sunset, or from sunrise to sunrise so that the length of the week (and the day) would be subject to slight variations depending upon the time of year and the observer's geographical latitude.}} so that, except when passing through [[daylight saving time]] transitions or [[leap second]]s,
:1 week = 7 days = 168 hours = 10,080 minutes = 604,800 seconds.
With respect to the [[Gregorian calendar]]:
*1 Gregorian calendar year = 52 weeks + 1 day (2 days in a [[leap year]])
*1 week = {{frac|1600|6957}} ≈ 22.9984% of an average Gregorian month

In a [[Gregorian calendar|Gregorian]] mean year, there are 365.2425 days, and thus exactly {{frac|52|71|400}} or 52.1775 weeks (unlike the [[Julian year (calendar)|Julian year]] of 365.25 days or {{frac|52|5|28}} ≈ 52.1786 weeks). There are exactly 20,871 weeks in 400 Gregorian years, so {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{#expr: {{CURRENTYEAR}} -400}} was a {{CURRENTDAYNAME}} just as was {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{CURRENTYEAR}}.

Relative to the path of the [[Moon]], a week is 23.659% of an average [[lunation]] or 94.637% of an average quarter lunation.

Historically, the system of [[dominical letter]]s (letters A to G identifying the weekday of the first day of a given year) has been used to facilitate [[determination of the day of the week|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]] [[Universal Time|UT]]):
Adding one to the [[remainder]] after dividing the Julian day number by seven (JD ''[[modulo operation|modulo]]'' 7 + 1) yields that date's [[ISO 8601]] day of the week. For example, the Julian day number of {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{CURRENTYEAR}} is {{#expr: floor({{CURRENTJULIANDAY}}+.5)}}. Calculating {{math|1={{#expr: floor({{CURRENTJULIANDAY}}+.5)}} mod 7 + 1}} yields {{#expr: (floor({{CURRENTJULIANDAY}}+.5) mod 7) + 1}}, corresponding to {{CURRENTDAYNAME}}<!-- +.5 because the JD is integer at noon UT-->.<ref>Richards, E. G. (2013). "Calendars". In S. E. Urban & P. K. Seidelmann, eds. ''Explanatory Supplement to the Astronomical Almanac'', 3rd ed. (pp. 585–624). Mill Valley, Calif.: University Science Books. 2013, pp. 592, 618.
This is equivalent to saying that JD0, i.e. 1 January 4713 BCE of the [[proleptic Julian calendar]], was a Monday.</ref> In 1973, [[John Horton Conway|John Conway]] devised the [[Doomsday rule]] for mental calculation of the weekday of any date in any year.