Julian Calendar

Julius Caesar introduced the calendar in 45 BCE on the advice of the Alexandrian astronomer Sosigenes, replacing the erratic pre-Julian Roman calendar with a fixed twelve month year averaging three hundred sixty five and a quarter days: three years of three hundred sixty five days followed by a leap year of three hundred sixty six, the extra day inserted after the twenty fourth of February. The reform was a genuine improvement on what preceded it, but the true solar year is slightly shorter than three hundred sixty five and a quarter days, so the calendar drifts against the seasons by about one day every hundred twenty eight years, a gap invisible across a single lifetime but substantial across centuries.

By the sixteenth century, when Pope Gregory XIII's reform corrected it, the Julian calendar's calculated spring equinox had slipped ten days from the astronomical one. The Gregorian reform's fix, dropping the leap day in century years not divisible by four hundred, is the whole difference between the two calendars, and the accumulated gap between them stands at thirteen days today, widening to fourteen after the Gregorian calendar skips its next leap year in 2100 while churches still keeping the Julian count do not.

The Julian calendar did not end with the Gregorian reform. It remains the liturgical calendar of the Russian Orthodox Church, the Serbian Orthodox Church, the Georgian Orthodox Church, the Jerusalem Patriarchate, and the monastic republic of Mount Athos, which is why Orthodox Christmas is commonly kept on the seventh of January by Gregorian reckoning, thirteen days after the twenty fifth of December the calendar itself still calls it.

Other Orthodox churches, including the Ecumenical Patriarchate of Constantinople and the Church of Greece, adopted a Revised Julian calendar in 1923 that realigns fixed feasts with the Gregorian dates until the two diverge again in 2800, while continuing to compute the movable feast of Easter by the older Julian based Alexandrian method regardless. The result is that Orthodox and Western Easter can fall on the same Sunday or several weeks apart depending on the year, purely as an artifact of which calendar underlies each computation.

The 1923 reform also produced a lasting split. Old Calendarist communities broke from the Church of Greece and other reforming churches specifically to keep the unrevised Julian calendar, and those communities remain active today, so the Julian calendar is not a single settled inheritance but the object of a live and ongoing ecclesiastical disagreement over which calendar a church should keep.