Anno Mundi is the official Byzantine calendar system, and it counts the years from Creation. Septuagint chronology sets Creation at 1 September 5509 BC. One year therefore runs from 1 September to 31 August.
The system first appears in the seventh-century work of the monk Georgios. In 988 it became the official calendar in Constantinople. Russia went on using it until 1700.
The indiction is a fifteen-year fiscal cycle. Its origins lie in the tax reassessments of Roman Egypt under Diocletian, who applied five-year cycles from AD 287 and a fifteen-year reassessment from 297. Under Constantine in 312-313 the fifteen-year cycle became a dating standard across the whole empire. Justinian made the indiction compulsory in all documents in 537.
In Byzantine practice the indiction year begins on 1 September. You divide the AM year by 15 and the remainder gives that year's indiction number. If the remainder is zero, the year counts as the 15th indiction.
Byzantium kept the rule laid down by the Council of Nicaea in 325. Easter falls on the first Sunday after the first full moon following the spring equinox. The Byzantine computation runs on the Julian calendar and the 19-year Metonic cycle.
Because the Orthodox world did not accept the Gregorian reform of 1582, Eastern and Western Easter most often fall on different dates. The current difference between the two calendars is 13 days.
Brumalia is a pagan winter festival of 24 days, celebrated between 24 November and 17 December. It was inherited from the cult of Dionysos and lived on for centuries inside Byzantine Christianity. The Council in Trullo banned it in 692, but people went on keeping it anyway.
Each day of the festival is dedicated to one letter of the Greek alphabet. Anyone whose name begins with that letter keeps the day as their own feast.
Byzantium preserved the structure of the Julian calendar and transliterated the Roman month names into Greek letters. Septemvrios, Oktovrios, Noemvrios, Dekemvrios, Ianouarios, Fevrouarios, Martios, Aprilios, Maios, Iounios, Ioulios, Avgoustos.
Dates are not reckoned by the Roman kalendae/nones/idus system. As in the Greek, Syriac and Egyptian traditions, they are counted forward from the beginning of the month.
The Julian calendar inserts a leap day every four years. That rule leaves a drift of roughly one day every 128 years against the tropical year. The Gregorian reform of 1582 closed the gap by dropping ten days.
Today the difference between the Julian and Gregorian calendars is 13 days. From 2100 onwards the difference will increase to 14 days.
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The fixed and movable feasts of the Byzantine liturgical year with their Greek names, and the twelve months. The year begins on 1 September with the indiction. Anno Mundi counts the years from the Creation.
Days whose date is the same every year.
Placed by their offset in days from Orthodox Easter.
The year opens on 1 September with the indiction.
Dates follow the Julian calendar. You can work out the Gregorian equivalent with the converter above.
About the Byzantine calendar
The Byzantine Anno Mundi ("year of the world") is a chronological era that counts years from the calculated date of the Creation, traditionally placed on 1 September 5509 BC in the proleptic Julian calendar. The figure was derived from a synthesis of biblical genealogies in the Septuagint by Byzantine chronographers, refined by writers such as Maximus the Confessor and Theophanes, and officially adopted under Justinian and his successors. It remained the principal dating system of the Byzantine Empire until 1453 and continued in use in the Eastern Orthodox world for centuries afterward.
The Indiction is a fifteen-year fiscal cycle, originally introduced by Diocletian in 297 CE for taxation purposes and retained throughout the Byzantine period as a standard chronological reference. Documents typically record both the year within the Indiction (1 to 15) and the consecutive Indiction number; combined with the Anno Mundi, this gave a precise and verifiable date. The Byzantine year, and therefore each Indiction, began on 1 September, the start of the Eastern Christian liturgical year, rather than on 1 January.
Eastern Orthodox Easter (Pascha) is computed using the rules adopted at the First Council of Nicaea in 325 CE, namely the first Sunday after the first ecclesiastical full moon on or after the vernal equinox, but with the equinox fixed at 21 March in the Julian calendar and the lunar epacts derived from the Alexandrian computus. Because the Julian calendar drifts against the tropical year by roughly one day every 128 years, Eastern Easter now falls between 4 April and 8 May in the Gregorian calendar, generally one to five weeks later than Western Easter. The Byzantine tradition preserves this computation in the Pascalion tables.
Brumalia was a Roman festival cluster celebrated from 24 November to 17 December, originally honouring Saturn, Bacchus, and the spirits of the dead during the period of shortest days (bruma = winter solstice). It survived robustly into Byzantine Constantinople, where each day of the festival was assigned to a letter of the Greek alphabet, and citizens whose names began with that letter were honoured. Although periodically condemned by ecclesiastical authorities (notably the Council of Trullo in 692 CE), Brumalia was still observed in the imperial palace as late as the tenth century, as documented by Constantine VII Porphyrogenitus in De Ceremoniis.
The Eastern Christian liturgical year begins on 1 September, a date inherited from the Byzantine fiscal calendar (the Indiction) and probably ultimately from the Seleucid civil year of late antique Syria. The choice was theologically reinterpreted: 1 September was identified as the day on which Christ entered the synagogue at Nazareth and read from the prophet Isaiah (Luke 4:16-21), inaugurating the year of the Lord's favour. The Byzantine Menaion, the liturgical book containing the fixed feasts of the year, therefore opens on 1 September with the feast of the Indiction, and concludes on 31 August.
The Byzantines retained the twelve Roman months but transliterated them into Greek: Septemvrios, Oktovrios, Noemvrios, Dekemvrios, Ianouarios, Fevrouarios, Martios, Aprilios, Maios, Iounios, Ioulios, and Avgoustos. The year, however, began with Septemvrios, in line with the liturgical calendar. These Greek month names spread with Byzantine influence across the Slavic Orthodox world, where they were either adopted directly or replaced by native Slavic names (snežen, lystopad, prosinec, and so on); both systems coexisted in many medieval chronicles.