Some countries, including Italy, Poland, Portugal, and Spain, accepted the Gregorian calendars introduction in 1582. The conversion took longer for other countries such as Great Britain (1752) and Lithuania (1915).
Moreover, some countries dropped a number of days when they began using the Gregorian calendar. For example, England and Scotland dropped 11 days for the conversion. Some communities did not accept the loss of these days and preferred to use the Julian calendar.
Many Orthodox churches still recognize the holiday dates according to the Julian calendar. The Julian calendar was revised in 1923 and this version is more in line with the Gregorian calendar. Some Orthodox churches follow the revised Julian calendar but many Orthodox churches still follow the more traditional Julian calendar, which has the original dates for Christian observances prior to the Gregorian calendars introduction. Also, the Orthodox New Year must not be confused with the start of the Eastern Orthodox liturgical calendar, which traditionally begins on September 1.
The Revised Julian calendar, also known as the Milanković calendar, or, less formally, New calendar, is a calendar, developed and proposed by the Serbian scientist Milutin Milanković 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 that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. The Revised Julian calendar temporarily aligned its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world.
The new calendar was proposed for adoption by the Orthodox churches at the Pan-Orthodox Congress of Constantinoplein May 1923 and subsequently it was adopted by several of the autocephalous Orthodox churches. The synod was chaired by controversial Patriarch Meletius IV of Constantinople, and called Pan-Orthodox by its supporters. But only the Patriarch of Constantinople and the Serbian Patriarch were represented. There were no representatives of the other members of the original Orthodox Pentarchy (the Patriarchates of Jerusalem, Antioch, and Alexandria) or from the largest Orthodox Church, the Russian Orthodox Church.
This 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 new calendar, thus dropping thirteen days. It then adopted the leap rule of Milanković, an astronomical delegate to the synod representing the Kingdom of Serbs, Croats and Slovenes.Milanković selected this rule because its mean year was within two seconds of the then current length of the mean tropical year.The present vernal equinox year, however, is about 12 seconds longer, in terms of mean solar days.
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′47.2″ E or UT+2h20m55s for the small 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 and the Estonian Orthodox Church which now use the Gregorian Easter).
