Gregorian calendar

The Gregorian calendar is the calendar widely used in the Western world. A modification of the Julian calendar, it was first proposed by the Neapolitan doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 (Note: The papal bull Inter gravissimas was signed in the year 1581 for unknown reasons, but printed on 1 March in 1582. Although 1581 is often attributed to the use of years beginning on 25 March by the papacy, other contemporaneous papal bulls have years that do not agree with March years, let alone years since a pope was named or other types of years.)

Numerical facts

When leap years, common years and different dates of Easter are taken into account, there are a total of 70 possible Gregorian calendars.

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An average year is 365.2425 days = 52.1775 weeks = 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.

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A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.

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A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.

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(Some years may also contain a leap second.)

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See also common year starting on Sunday and dominical letter.

Related Topics:
Common year starting on Sunday - Dominical letter

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The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks. So, for example, the days of the week in Gregorian 1603 were exactly the same as for 2003. This also causes more months to begin on a Sunday (and hence have Friday 13) than any other day of the week. 688 out of every 4800 months (or 172/1200) begin on a Sunday, while only 684 out of every 4800 months (171/1200) begin on each of Saturday and Monday, the least common cases.

Related Topics:
1603 - 2003 - Friday 13

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A smaller cycle is 28 years (1,461 weeks), provided that there is no dropped leap year in between. Days of the week in years may also repeat after 6, 11, 12, 28 or 40 years. Intervals of 6 and 11 are only possible with common years, while intervals of 28 and 40 are only possible with leap years. An interval of 12 years can occur with either type, but only when there is a dropped leap year in between.

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An algorithm called the Doomsday algorithm is a method by which you can discern which of the 14 calendar variations should be used in any given year (after the Gregorian reformation). It is based on the last day in February, referred to as the Doomsday.

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