Gregorian calendar

==Description==
The Gregorian calendar, like the [[Julian calendar]], is a [[solar calendar]] with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a [[leap day]] being added to February in the [[leap year]]s. The months and length of months in the Gregorian calendar are the same as for the Julian calendar. The only difference is that the Gregorian reform omitted a leap day in ''three'' centurial years every 400 years and left the leap day unchanged.

A leap year normally occurs every four years: the leap day, historically, was inserted by doubling 24 February{{snd}} there were indeed [[bissextus|two days dated 24 February]]. However, for many years it has been customary to put the extra day at the end of the month of February, adding a [[29 February]] for the leap day. Before the [[Mysterii Paschalis|1969 revision]] of its [[General Roman Calendar]], the Catholic Church delayed February feasts after the 23rd by one day in leap years; masses celebrated according to the previous calendar still reflect this delay.{{sfnp|Richards|1998|page=101}}

{| class="wikitable"
|+A year is divided into twelve [[month]]s
! {{abbr|No.|Number}} !! Name !! Length in days
|-
| 1 || January || 31
|-
| 2 || February || 28 (29 in [[leap year]]s)
|-
| 3 || March || 31
|-
| 4 || April || 30
|-
| 5 || May || 31
|-
| 6 || June || 30
|-
| 7 || July || 31
|-
| 8 || August || 31
|-
| 9 || September || 30
|-
| 10 || October || 31
|-
| 11 || November || 30
|-
| 12 || December || 31
|}

Gregorian years are identified by consecutive year numbers.<ref>Clause 3.2.1 [[ISO 8601]]</ref> A calendar date is fully specified by the year (numbered according to a [[calendar era]], in this case ''[[Anno Domini]]'' or [[Common Era]]), the month (identified by name or number), and the day of the month (numbered sequentially starting from 1). Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar (see the [[#Beginning of the year|"beginning of the year"]] section below).
{{clear}}

Calendar cycles repeat completely every 400 years, which equals 146,097 days.{{efn|The cycle described applies to the solar, or civil, calendar. If one also considers the ecclesiastical lunar rules, the lunisolar Easter ''computus'' cycle repeats only after 5,700,000 years of 2,081,882,250 days in 70,499,183 lunar months, based on an assumed mean lunar month of 29 days 12 hours 44 minutes {{sfrac|2|49928114|70499183}} seconds. (Seidelmann (1992), p. 582) [To properly function as an Easter ''[[computus]]'', this lunisolar cycle must have the same mean year as the Gregorian solar cycle, and indeed that is exactly the case.]}}{{efn|The extreme length of the Gregorian Easter ''computus'' is due to its being the product of the 19-year [[Metonic cycle]], the thirty different possible values of the [[epact]], and the least common multiple (10,000) of the 400-year and 2,500-year solar and lunar correction cycles.{{sfnp|Walker|1945|page=218}}}} Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is {{sfrac|365|97|400}} days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds.{{efn|The same result is obtained by summing the fractional parts implied by the rule: {{nowrap|1= 365 + {{sfrac|1|4}} − {{sfrac|1|100}} + {{sfrac|1|400}} = 365 + 0.25 − 0.01 + 0.0025 = 365.2425}}}} During intervals that do not contain any century common years (such as 1800, 1900 and 2100), the calendar repeats every 28 years, during which 29 February will fall on each of the seven days of the week once and only once. All other dates of the year fall on each day exactly four times, each day of the week having gaps of 6 years, 5 years, 6 years, and 11 years, in that order.