The Gregorian calendar is the official solar calendar of almost every country in the world.
It takes its name from Pope Gregory XIII, who introduced it on 4 October 1582 with the papal bull Inter gravissimas, promulgated in Villa Mondragone (near Monte Porzio Catone, RM).
It is a calendar based on the solar year, i.e. on the cycle of the seasons, which corrects the old Julian calendar in force since 46 BC. to 1582. The year is made up of 12 months with different durations (from 28 to 31 days) for a total of 365 or 366 days: the year of 366 days is called a leap year. This repetition occurs every four years, with some exceptions (see below for the rule).
Other countries such as Iran, Afghanistan, Eritrea, Ethiopia, Nepal, India, Japan, North Korea, Bangladesh, Israel, Pakistan, Taiwan, Thailand and Burma also combine the Gregorian calendar with a local calendar.
Duration of months
The months of the Gregorian calendar are:
January (31 days)
February (28 days, 29 in leap year)
March (31 days)
April (30 days)
May (31 days)
June (30 days)
July (31 days)
August (31 days)
September (30 days)
October (31 days)
November (30 days)
December (31 days) The days of each month are identified by a progressive numbering, starting from 1. The first day of the year is January 1st, while the last is December 31st.
There are several ways to easily remember the length of the months. For example, it is enough to remember that in a year there are only two consecutive months (July and August) of 31 days. Another method uses this nursery rhyme:
Thirty days count November with April, June and September twenty-eight there is one all the others have thirty-one.
The most modern variant of which is:
Thirty days has November with April, June and September twenty-eight there is one all the others have thirty-one.
Another uses the knuckles of the hand and the depressions between them. The knuckles will indicate the "long" months (31 days), the hollows the "short" months (28, 29 or 30 days). Starting from a lateral knuckle, "January" is struck; the adjacent valley indicates "February"; the next knuckle indicates "March" and so on until "July" (last knuckle). At this point it is necessary to start again from the first knuckle (and not go back) by beating "August" and then "September" in the valley continuing until "December".
There is also a relationship between the succession of months from "January" to "December" and the piano keys from the note "fa" to the note "mi": the white keys correspond to the months of 31 days, the black keys to those of shorter duration .
Numbering of the years
The origin of the years of the Gregorian calendar is obviously the same as that of the Julian calendar. Year 1 is one that begins seven days after the traditionally assumed date of Christ's birth. The era of the Gregorian calendar, therefore, is also called either the Era of the Nativity / Incarnation or more simply the Vulgar Era and the years can be followed by the abbreviation A.D. (for "after Christ") or e.v. (for "it was vulgar").
Note that the Gregorian calendar has been in effect from 1582 onwards, therefore, unless otherwise noted, historians use the Julian calendar dates for all events prior to its entry into force. When using the Gregorian calendar to date events prior to 1582, it is said that the proleptic Gregorian calendar is being used.
According to the Julian calendar, the years whose numbering is a multiple of 4 are leap years: the average Julian year therefore lasts 365 days and 6 hours (the average of three years of 365 days and one of 366). This duration does not correspond exactly to that of the average solar year, which is obtained from astronomical observations: the latter is in fact shorter by 11 minutes and 14 seconds. Consequently, the Julian calendar accumulates approximately one day behind every 128 years with respect to the passing of the seasons: enter 32