The Revised Julian calendar or, less formally, New Calendar, is a calendar scheme, originated in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar scheme that has come to predominate worldwide. In 2800 the two schemes will diverge again, though more slowly than the Julian and Gregorian do.
The term "Revised Julian" is informative primarily in describing the fact that it replaces the de facto Orthodox endorsement of the Julian scheme, and has the effect of avoiding any implicit recognition of Pope Gregory XIII's promulgation of a system with the same goals and general approach in the Gregorian reform of 1582.
The Revised Julian calendar was proposed for adoption by the Orthodox churches at a synod in Constantinople in May 1923. The synod synchronized the new calendar with the Gregorian calendar by specifying that the next 1 October of the Julian calendar would be 14 October in the Revised Julian calendar, thus dropping thirteen days. It then adopted a leap year rule that differs from that of the Gregorian calendar: Years evenly divisible by four are leap years, except that years evenly divisible by 100 are not leap years, unless they leave a remainder of 200 or 600 when divided by 900, then they are leap years. This means that the two calendars will first differ in 2800, which will be a leap year in the Gregorian calendar, but a common year in the Revised Julian calendar. This leap year rule was proposed by the Serbian scientist Milutin Milanković, an astronomical delegate to the synod representing the Kingdom of Serbs, Croats and Slovenes.
Milanković selected this rule, which produces an average year length of 365.242222… days, because it was within two seconds of the then current length of the mean tropical year. However, the current vernal equinox year is slightly longer, about half-way between the two lengths, so for a few thousand years the Revised Julian calendar will do a marginally worse job than the Gregorian calendar at keeping the vernal equinox on March 21; it will be on March 22 more often than the Gregorian calendar will put it on March 20. However the Revised Julian calendar is more accurate regarding the length of the mean tropical year when compared to Gregorian calendar. But the length of a day is increasing by about 1.7 milliseconds per century (see tidal acceleration), so the number of days per year decreases by about 0.0001 each millennium. This means that in the long run, the Revised Julian calendar will also be inaccurate even if the mean tropical year is the basis.
The synod also proposed the adoption of an astronomical rule for Easter: Easter was to be the Sunday after the midnight-to-midnight day at the meridian of the Church of the Holy Sepulchre in Jerusalem (35°13'46"E or UT+2h20m55s for the large dome) during which the first full moon after the vernal equinox occurs. Although the instant of the full moon must occur after the instant of the vernal equinox, it may occur on the same day. If the full moon occurs on a Sunday, Easter is the following Sunday. However, all Eastern Orthodox churches rejected this rule and continue to use the Julian calendar to determine the date of Easter (except for the Finnish Orthodox Church, which now uses the Gregorian Easter).
The Revised Julian calendar was adopted by the Orthodox churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, and Bulgaria (the last in 1963), called the New calendarists. It was rejected by the Orthodox churches of Jerusalem, Russia, Serbia (including later on the resurrected, though uncanonical Macedonian Orthodox Church), Georgia, and the Greek Old Calendarists. Although Milanković stated that the Russian Orthodox Church adopted the Revised Julian calendar in 1923, the present church continues to use the Julian calendar for both its fixed festivals and for Easter. A solution to this conundrum is to hypothesize that it was accepted only by the short-lived schismatic Renovationist Church, which had seized church buildings with the support of the Soviet government while Patriarch Tikhon was under house arrest. After his release, on 15 July 1923, he declared that all Renovationist decrees were without grace, presumably including its acceptance of the Revised Julian calendar.
While the Revised Julian calendar has been adopted by many of the smaller national churches, a majority of Orthodox Christians continue to adhere to the traditional Julian Calendar, and there has been much acrimony between the two parties over the decades since the change, leading sometimes even to violence, especially in Greece.
Critics see the change in calendar as an unwarranted innovation, influenced by Western society. They say that no sound theological reason has been given for changing the calendar, that the only reasons advanced are social.
The argument is also made that since the use of the Julian Calendar was implicit in the decision of the First Ecumenical Council at Nicea (325) which standardized the calculation of the date of Pascha (Easter), no authority less than an Ecumenical Council may change it. The adoption of a new calendar has broken the unity of the church, undoing the whole purpose of the council of Nicea, so once again, "on the same day some should be fasting whilst others are seated at a banquet."
Liturgical objections to the New Calendar stem from the fact that it adjusts only those liturgical celebrations that occur on fixed calendar dates, leaving all of the commemorations on the moveable cycle on the original Julian Calendar. This upsets the harmony and balance of the liturgical year. This disruption is most noticeable during Great Lent. Certain feast days are designed to fall during Lent, such as the feast of the Forty Martyrs of Sebaste. The Feast of the Annunciation is also intended to fall either before Pascha or during Bright Week. Sometimes, Annunciation will fall on the day of Pascha itself, a very special concurrence known as Kyrio-Pascha, with special liturgical practices appointed for such an occurrence. However, under the Revised Julian calendar, Kyrio-Pascha becomes an impossibility. The Apostles' Fast displays the most difficult aspect of the Revised Julian calendar. The fast begins on the moveable cycle and ends on the fixed date of June 29; since the Revised Julian calendar is 13 days ahead of the traditional Julian calendar, the Apostles' Fast is 13 days shorter for those who follow the New Calendar, and some years it is completely abrogated. Furthermore, critics of the New Calendar point out the definite advantage to celebrating Nativity separately from the secular observance of Christmas and New Year with its use of alcohol and ribald partying.
Critics also point out that proponents of the New Calendar tend to use worldly rather than spiritual justification for changing the calendar: wanting to "party with everyone else" at Christmas; concern that the gradual shift in the Julian Calendar will somehow negatively affect the celebration of feasts that are linked to the seasons of the year. However, opponents counter that the seasons are reversed in the southern hemisphere, where the liturgical celebrations are no less valid. Proponents also argue that the Revised Julian Calendar is somehow more "scientific", but opponents argue that science is not the primary concern of the Church; rather, the Church is concerned with other-worldliness, with being "in the world, but not of it", fixing the attention of the faithful on eternity. Scientifically speaking, neither the Gregorian Calendar nor the Revised Julian is absolutely precise. This is because the solar year cannot be evenly divided into 24 hour segments. So any public calendar is imprecise; it is simply an agreed-upon designation of days.