The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). 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 (also known as the Berbers).^[1]

The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar, as a reform of the earlier Roman calendar, which was largely a lunisolar one.^[2] It took effect on 1 January 45 BC, by his edict. Caesar's calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years, until 1582 when Pope Gregory XIII promulgated a revised calendar.

The Julian calendar has two types of years: a normal year of 365 days and a leap year of 366 days. They follow a simple cycle of three normal years and one leap year, giving an average year that is 365.25 days long. That is more than the actual solar year value of approximately 365.2422 days (the current value, which varies), which means the Julian calendar gains one day every 129 years. In other words, the Julian calendar gains 3.1 days every 400 years.

Gregory's calendar reform modified the Julian rule, to reduce the average length of the calendar year from 365.25 days to 365.2425 days and thus corrected the Julian calendar's drift against the solar year: the Gregorian calendar gains just 0.1 day over 400 years. For any given event during the years from 1901 through 2099, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date (for instance Julian 1 January falls on Gregorian 14 January). Most Catholic countries adopted the new calendar immediately; Protestant countries did so slowly in the course of the following two centuries or so; most Orthodox countries retain the Julian calendar for religious purposes but adopted the Gregorian as their civil calendar in the early part of the twentieth century.

The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days.^[5] Some say the mensis intercalaris always had 27 days and began on either the first or the second day after the Terminalia (23 February).^[6]

If managed correctly this system could have allowed the Roman year to stay roughly aligned to a tropical year. However, since the pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.^[7]

Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.^[8] A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date,^[9] this stability had become an ordinary fact of life.

Although the approximation of 365+1⁄4 days for the tropical year had been known for a long time,^[10] ancient solar calendars had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.

The octaeteris, a cycle of eight lunar years popularised by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Athens, is 1.53 days longer than eight mean Julian years. The length of nineteen years in the cycle of Meton was 6,940 days, six hours longer than the mean Julian year. The mean Julian year was the basis of the 76-year cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle.

In Persia (Iran) after the reform in the Persian calendar by introduction of the Persian Zoroastrian (i. e. Young Avestan) calendar in 503 BC and afterwards, the first day of the year (1 Farvardin=Nowruz) slipped against the vernal equinox at the rate of approximately one day every four years.^[11][12]

Likewise in the Egyptian calendar, a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238 BC (Decree of Canopus). Caesar probably experienced this "wandering" or "vague" calendar in that country. He landed in the Nile delta in October 48 BC and soon became embroiled in the Ptolemaic dynastic war, especially after Cleopatra managed to be "introduced" to him in Alexandria.

Caesar imposed a peace, and a banquet was held to celebrate the event.^[13] Lucan depicted Caesar talking to a wise man called Acoreus during the feast, stating his intention to create a calendar more perfect than that of Eudoxus^[13] (Eudoxus was popularly credited with having determined the length of the year to be 365+1⁄4 days).^[14] But the war soon resumed and Caesar was attacked by the Egyptian army for several months until he achieved victory. He then enjoyed a long cruise on the Nile with Cleopatra before leaving the country in June 47 BC.^[15]

Caesar returned to Rome in 46 BC and, according to Plutarch, called in the best philosophers and mathematicians of his time to solve the problem of the calendar.^[16] Pliny says that Caesar was aided in his reform by the astronomer Sosigenes of Alexandria^[17] who is generally considered the principal designer of the reform. Sosigenes may also have been the author of the astronomical almanac published by Caesar to facilitate the reform.^[18] Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the 365+1⁄4 days of Greek astronomy. According to Macrobius, Caesar was assisted in this by a certain Marcus Flavius.^[19]

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Caesar's reform only applied to the Roman calendar. However, in the following decades many of the local civic and provincial calendars of the empire and neighbouring client kingdoms were aligned to the Julian calendar by transforming them into calendars with years of 365 days with an extra day intercalated every four years.^[20][21] The reformed calendars typically retained many features of the unreformed calendars. In many cases, the New Year was not on 1 January, the leap day was not on the traditional bissextile day, the old month names were retained, the lengths of the reformed months did not match the lengths of Julian months, and, even if they did, their first days did not match the first day of the corresponding Julian month. Nevertheless, since the reformed calendars had fixed relationships to each other and to the Julian calendar, the process of converting dates between them became quite straightforward, through the use of conversion tables known as "hemerologia".^[22]

The three most important of these calendars are the Alexandrian calendar and the Ancient Macedonian calendar─which had two forms: the Syro-Macedonian and the 'Asian' calendars. Other reformed calendars are known from Cappadocia, Cyprus and the cities of (Roman) Syria and Palestine. Unreformed calendars continued to be used in Gaul (the Coligny calendar), Greece, Macedon, the Balkans and parts of Palestine, most notably in Judea.

The Asian calendar was an adaptation of the Ancient Macedonian calendar used in the Roman province of Asia and, with minor variations, in nearby cities and provinces. It is known in detail through the survival of decrees promulgating it issued in 8 BC by the proconsul Paullus Fabius Maximus. It renamed the first month Dios as Kaisar, and arranged the months such that each month started on the ninth day before the kalends of the corresponding Roman month; thus the year began on 23 September, Augustus's birthday.