The Eastern churches celebrate the beginning of the church year (indiction) in September: on 1 September according to the Gregorian Calendar and on 14 September according to the Julian Calendar. The full religious year lasts from 14 (1) September to 13 September (31 August)
The date of the new church year was adopted by the fathers of the Ecumenical Council in Nicea in 325. The Eastern churches still live according to that decree. As for the Roman Catholic Church, it begins the liturgical year on the first Sunday of Advent, the Christmas fast.
The indiction by which the fathers of the Council in Nicea established the date of the celebration of the beginning of the church year is called "Byzantine," "Constantinople" or Constantine's. In the beginning the indiction was obligatory for the whole Roman Empire, except Egypt. Emperor Justinian I decreed that all official documents should be dated after the indict. The Roman Church under Pope Pelagius II approved the indict for its use for the dates of documents and ceased to fulfil that resolution only in 1097.
The day of the beginning of the church year became a church holiday. It is not known exactly when the beginning of the indiction became a church holiday but it was already in existence in the 8th century.
Fr. Yulian (Katrii) in his book "Know your rite," notes: "the church liturgical year is a powerful hymn of honour and glory to God in which the triple church participates: the Church Triumphant in Heaven, the Church Militant on Earth and the Church Suffering in purgatory." "The whole content of our Holy Faith finds its best description in it," the author adds.