The greatest of these days, of course, is Easter. As can be seen from their titles, the major holy days commemorate various events in the lives of Jesus Christ and the Holy Virgin Mary.
The second group of holy days commemorate various events in the lives of Prophets, Apostles, Fathers of the Orthodox Church, Martyrs, Saints and all others who have aided the propagation of Christianity and the Orthodox religion, and certain events in the life of the Church. The holy days are also divided into movable feasts, which have no definite days or even months appointed for their celebration and the non movable feasts, which are always celebrated on definite days of the year.
As much as the Orthodox Church follows the lunar system in reckoning of days of the year (as is also true of the Hebrew system in the Old Testament), Easter is celebrated on varying dates from year to year. Therefore, all holy days such as the Entry of Our Lord into Jerusalem, Good Friday, Ascencion and Pentecost, which are dependent upon Easter, are likewise celebrated on varying dates.
The Canon Law concerning the celebration of Easter is as follows: "If any bishop, or priest, or deacon, celebrates the Holy Day of Easter before the spring equinox or together with the Hebrews or before them, he is to be EXCOMMUNICATED from the Church" (Apostolic Canon No. 7).
A Council of the Church that met at Antioch reaffirmed in its first Canon this Canon of the Apostles. Therefore, Easter can't be celebrated earlier than The 21st of March or later than The 25th of May. If our Easter coincides with the Hebrew Passover, we have to postpone Easter a few days or even a week.
If the spring equinox comes before the Hebrew Passover, we celebrate our Easter on the Sunday, following the next spring full moon, or five weeks later. The Orthodox Church adheres strictly to this rule, so as not to be excommunicated from the Body of the Church. This may be seen from the fact that the Greek Orthodox Church, which celebrates its set holy Days according to the new Gregorian Calendar, celebrated Easter, nevertheless, according to Canon Law and the Julian Calendar.
We have, therefore, in our Orthodox Church three distinct calendars. The First is the Easter Calendar, for our series of Gospels and Epistles begin with Easter. The succession of the singing of the Eight Tones likewise begins with Easter. The Second is the Civil Calendar. In our Church this is still the old Julian calendar. The difference between the Julian and the New or Gregorian Calendar is at present thirteen days. The Third is our Church Calendar. We seldom refer to this Calendar, but it is from it we reckon our Holy Days.
The Church Calendar begins with the Birth of the Holy Virgin Mary (Theotokos) on September the 8th according to the Old or Julian Calendar and September 21st according to the new or Gregorian Calendar. This Holy Day corresponds closely to the date of the Jewish New Year. In its Holy Days, our Church has generally followed closely the celebrations of the Hebrews. To them the New Year signified the beginning of the new Era in their lives. Similarly, the Early Corinthian Church proclaimed the new Era by announcing to the world the Birth of the Holy Virgin Mary, who was to be the mother of Our Lord Jesus Christ, Who in turn, by His Birth, Was to bring a new Era into the world: the Era of Christianity.
To each of the Holy Days our Church gave the texts of the Troparion and Kontakion proper to that particular day, along with a brief description of that Day and its significance. The Troparion and the Kontakion in themselves offer succinct summaries of the Day. Whereas the greater Holy Days are presented in the order in which the events occurred, the lesser Holy Days are arranged according to their position in the Calendar.