This is due to the fact that the worship services in the Orthodox Church are harmoniously integrated into a single whole, consisting of several parts. Generally speaking, they can be called divine service cycles: a daily divine service cycle, a weekly one, and an annual one. In addition, there are also divine services for the Lenten Triodion (weeks of Lent and several weeks that precede it) and the Pentecostarion, which includes the period from Easter to Holy Trinity Day and All Saints’ Sunday.
Scripture is read as part of these divine service cycles. This article will talk about the New Testament fragments read at the Liturgy, that is, excerpts from the Epistle and the Gospel.
Every day there is a regular passage of the day: certain fragments of the Epistle and the Gospel that follow one another. Thus the entire Epistle and the entire Gospel are read out in succession during the ecclesiastical year. This is the unchangeable daily portion. It can be compared to sunrise and sunset, the change of day and night. It is something indelible. All the Holy Scripture of the New Testament (except the Apocalypse) is read in one year. So, year after year, century after century, millennium after millennium, the Church continues to live and feed the world with the life-giving word of Holy Scripture.
Now about the second passage. There is also the Church calendar, which includes the days of saints. Many of them, especially those who are marked by polieley services and All-Night Vigils, have been designated certain Epistle and Gospel readings. In such cases, they are read after the aforementioned readings of the day. For example, this year, the memory day of the holy First Apostles Peter and Paul coincided with a Sunday afternoon. Therefore, we read two pairs of New Testament passages: the first two for the Sunday (this service is more important than that for the Apostles), and then another pair of readings for the Holy Apostles Peter and Paul.
Reading two pairs of Epistle and Gospel passages is a fairly common phenomenon in the practice of the Church. Three pairs are far less common. Though they are rare, they do happen. For example, Orthodox Christians celebrate the Nativity of the Holy Glorious Prophet, Forerunner and Baptist John on July 7. For the sake of this great feast, which is entitled to an All-Night Vigil with a litya, the reading of the day is omitted and shifted to the previous day, July 6. Thus, on July 6 (this year it was Monday), at the Liturgy, the reading of the day went first, followed by the reading of Tuesday. But also on that day the polieley to the Vladimir Icon of the Mother of God is due. That is why after two readings of the day there was also a reading of the Epistle and Gospel passages dedicated to the Most Holy Mother of God. All in all, there were three passages of the Epistle and three passages of the Gospel.
Someone might say that you Orthodox people are very complicated and confusing. That’s not true. Believe me as a priest. The more you penetrate into the wonderful world of Liturgy, Orthodox worship, the more you are amazed by its extraordinary beauty and harmony. It does not seem chaotic. On the contrary, you can feel the whole universe, which consists of the Church of Heaven and the Church of Earth and which begins at the Throne of God. Our worship service is a breakthrough into Heaven, the unity with the spiritual world, which is the Church, where Angels and people, the living and the dead, glorify God who abides in our midst.
Translated by The Catalogue of Good Deeds
Source: https://pravlife.org/ru/content/pochemu-v-opredelennye-dni-na-bogosluzheniyah-chitaetsya-odno-zachalo-svyashchennogo