The calendar
A calendar is an overview of the days, weeks and months of a year. The word “calendar” comes from the Latin calendarium (debt book), where the days on which debts and interest had to be paid were noted.
The rules for setting up calendars result from astronomical circumstances (lunar phases, solar year) and corresponding calendar calculations.
The year
In the astronomical sense, various definitions of the term “year” are based on the duration of the Earth's revolution around the sun, or the apparent movement of the sun in the sky. A distinction is made between sidereal year, tropical year and anomalistic year, which all last around 365 ¼ days with minor mutual deviations.
The civil year or calendar year is based on the tropical year. It lasts either 365 days (common year) or 366 days (leap year). In everyday life, the term “year” means the time from January 1st to December 31st.
The Gregorian calendar
The Gregorian calendar, also known as the civil calendar, is the most widely used calendar worldwide. It came about at the end of the 16th century through a reform of the Julian calendar. It is named after Pope Gregory XIII, who decreed it in 1582 with the papal bull Inter gravissimas. Over time it replaced both the Julian and numerous other calendars.
Like the Julian calendar, the Gregorian calendar is a solar calendar but with an improved leap year rule. There is currently a difference of 13 days, so in many churches Christmas falls on January 7th.
The month
The month is the period of 28, 29, 30 or 31 days. The term "month" refers to the Earth's moon.
Depending on the definition, the lunar month lasts between just under 27⅓ and a good 29½ days. On average these are, e.g. B. from full moon to full moon 29.530589 d (29 days, 12 hours, 44 minutes, 2.9 seconds). Accordingly, it is not easy to have a wheel clock display the phases of the moon.
January (Hartung) is dedicated to the Roman god Janus. It is shown as having two heads, so it looks forwards and backwards in keeping with the change from shorter to longer days (daytime brightness).
February (Hornung): Latin februare “to clean”. February was the Roman festival of purification and atonement before the beginning of the new year.
March (Lenzmond) is the first month of the Roman calendar and named after the god of war and vegetation, Mars. This beginning of the year corresponded to the resurgence of nature.
April (Easter Moon) is derived from the Latin aperire “to open”. Nature is blooming.
May (Happy Moon) is named after the Roman goddess Maia.
June (Brachmond) is named after the queen of the gods Juno.
July (Hay Moon) is the birth month of Gaius Julius Caesar.
August (Harvest Moon) is the month of the death of Emperor Augustus.
September (autumn moon) is the seventh month (Latin septem = “seven”), so March was originally the first month. Accordingly, the months October (wine moon) follow as the eighth month (Latin octo = "eight"), November (winter moon) as the ninth month (Latin novem = "nine" ) and December (Christmas moon) the tenth month (Latin decem = “ten”) in the Roman calendar.
The days of the week
The week is now a common time unit of seven days in almost all cultures. Since January 1, 1976, the week according to ISO standard 8601 begins on Monday and ends on Sunday.
Graphic: Weekday heptagram with the symbols of the days of the week: Sun (Sunday, top), Moon (Monday, bottom right) and further along the green solid line to Mars (Tuesday - Tius), Mercury (Wednesday - Wodan ), Jupiter (Thursday - Thor), Venus (Friday - Freya), Saturn (Saturday)
