The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year. The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people.
A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronised with the astronomical year or seasonal year. Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.
The Revised Julian calendar, or less formally the new calendar and also known as the Milanković calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar was still in use by all of the Eastern Orthodox Churches and affiliated nations, while the Catholic and Protestant nations were using the Gregorian calendar. Thus, Milanković's aim was to discontinue the divergence between the naming of dates in Eastern and Western churches and nations. It was intended to replace the Julian calendar in Eastern Orthodox Churches and nations. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which had been proclaimed in 1582 by Pope Gregory XIII.
The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 8 when the quadrennial leap year stabilized. The leap years that were actually observed between the implementation of the Julian calendar in 45 BC and AD 8 were erratic.
The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. It was used for fiscal purposes in Egypt until the adoption of the Gregorian calendar on 11 September 1875. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III which consisted of adding an extra day every fourth year. However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic or Alexandrian calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.
As a moveable feast, the date of Easter is determined in each year through a calculation known as computus. Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March. Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. The complexity of the algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified.
The Swedish calendar or Swedish style was a calendar in use in Sweden and its possessions from 1 March 1700 until 30 February 1712. It was one day ahead of the Julian calendar and ten days behind the Gregorian calendar. Easter was calculated astronomically, with a minor exception, from 1740 to 1844.
The epact used to be described by medieval computists as the age of a phase of the Moon in days on 22 March; in the newer Gregorian calendar, however, the epact is reckoned as the age of the ecclesiastical moon on 1 January. Its principal use is in determining the date of Easter by computistical methods. It varies from year to year, because of the difference between the solar year of 365–366 days and the lunar year of 354–355 days.
Dominical letters or Sunday letters are a method used to determine the day of the week for particular dates. When using this method, each year is assigned a letter depending on which day of the week the year starts. The Dominical letter for the current year 2024 is GF.
Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.
The Doomsday rule, Doomsday algorithm or Doomsday method is an algorithm of determination of the day of the week for a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years. The algorithm for mental calculation was devised by John Conway in 1973, drawing inspiration from Lewis Carroll's perpetual calendar algorithm. It takes advantage of each year having a certain day of the week upon which certain easy-to-remember dates, called the doomsdays, fall; for example, the last day of February, 4/4, 6/6, 8/8, 10/10, and 12/12 all occur on the same day of the week in any year.
Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, this is the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923.
Inter gravissimas was a papal bull issued by Pope Gregory XIII on 24 February 1582. The document, written in Latin, reformed the Julian calendar. The reform came to be regarded as a new calendar in its own right and came to be called the Gregorian calendar, which is used in most countries today.
The solar cycle is a 28-year cycle of the Julian calendar, and 400-year cycle of the Gregorian calendar with respect to the week. It occurs because leap years occur every 4 years, typically observed by adding a day to the month of February, making it February 29th. There are 7 possible days to start a leap year, making a 28-year sequence.
The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.
The tables below list equivalent dates in the Julian and Gregorian calendars. Years are given in astronomical year numbering.
The Hanke–Henry Permanent Calendar (HHPC) is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars that maintain synchronization with the solar year by intercalating entire weeks rather than single days. It is a modification of a previous proposal, Common-Civil-Calendar-and-Time (CCC&T). With the Hanke–Henry Permanent Calendar, every calendar date always falls on the same day of the week. A major feature of the calendar system is the abolition of time zones.
A perennial calendar is a calendar that applies to any year, keeping the same dates, weekdays and other features.
The adoption of the Gregorian Calendar was an event in the early modern history of most cultures and societies, marking a change from their traditional dating system to the modern dating system – the Gregorian calendar – that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between. A few still have not, but except for these, the Gregorian calendar is now the world's civil calendar universally, although in many places an old style calendar remains used in religious or traditional contexts. During – and for some time after – the change between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.
A century common year is a common year in the Gregorian calendar that is divisible by 100 but not by 400. Like all common years, these years do not get an extra day in February, meaning they have 365 days instead of 366. These years are the only common years that are divisible by 4.
