I do a lot of posts that touch upon the calendar – seasons, celebrations, holidays, the Moon, astronomical occurrences. Most of the time I am in “our calendar” – the Gregorian calendar. Today, it is the internationally accepted civil calendar and is also known as the “Western calendar” or “Christian calendar”. It was named after the man who first introduced it in February 1582: Pope Gregory XIII.
It is a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. 11 months have either 30 or 31 days, while February has only 28 days during the common year. (Nearly every 4 years is a Leap Year, when one extra – or intercalary – day is added on 29 February.)
The Gregorian calendar reformed the Julian calendar because the Julian calendar introduced an error of 1 day every 128 years. The introduction of the Gregorian calendar allowed for a realignment with astronomical events like equinoxes and solstices.
However this switch from Julian to Gregorian probably was pretty odd to folks back in 1582 in Italy, Poland, Portugal and Spain where it was first adopted. This “Gregorian reform” meant a number of days had to be dropped. They dropped 10 days in October 1582. It changed the rules to determine the date of Easter, and for calculating Leap Years.
Still, the Gregorian calendar is not the only one.
The Hebrew calendar has months that change with the new Moon, and the full Moons fall in the middle of the month.
A solar year is about 11 days longer than twelve lunar months, so to keep holidays tied to their seasons, the Hebrew calendar occasionally repeats the month of Adar.
The Islāmic calendar is also lunar. The months start with the first sighting of the waxing crescent Moon, a few days after the New Moon.
Unlike the Gregorian and the Hebrew calendars, the Islāmic calendar has no leap days or leap months to stay in sync with the seasons, so Islāmic holidays occur approximately 11 days earlier each solar year.