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Ukrainian Orthodox Church
of St. George
The Julian “Old” Calendar
The Julian Calendar was created by Julius Caesar in the first century BC. While it was an improvement on the previous calendar in use, it was flawed in that it would lose a day approximately every 150 years. It was the standard calendar known for the world until 1582, when Pope Gregory created a new calendar, designed to correct the loss of days in the Julian Calendar. Today, the "Gregorian" Calendar is the standard calendar used by most of the world. Up until the 1920's all the Orthodox world chose to maintain the Julian Calendar for marking the holy days of the Church year.
Today, there is a 13-day difference between the two calendars. For example, when it is January 14th on the Gregorian Calendar, it is only January 1st on the Julian Calendar.
The Revised “New” Julian Calendar
Since the 1920's, a number of Orthodox jurisdictions, beginning with the Greek Orthodox Church, have switched to a revised version of the Julian calendar. On this calendar, feasts that fall on the same day every year ("immovable" feasts like Christmas) are celebrated on the Gregorian Ascension. Pentecost and All Saints' Day - the Sunday after Pentecost) are celebrated according to the Julian configuration. About half of the Orthodox jurisdictions in North America (among them the Greeks, Antiochians, Romanians and the Orthodox Church in America) follow the revised Julian Calendar. Most of the Slavic Orthodox jurisdictions and a small group of Greek Orthodox follow the standard Julian calendar.
Source: The Ukrainian Orthodox Church of Canada