Explanation of Vespers
Saint Clement received from his teacher, Saint Peter the Apostle, all the doctrines of faith, the rites of worship, their noble offices and services and their holy prayers. Hence he commands us saying, "The offerings and services must not be fulfilled as they come, but in order, and at set times and hours."

The day's beginning was computed from the evening, as it is said in the Holy Book, "And it was evening, and it was morning, one day."(Gen 1:5) Hence, the Church considers the beginning of her day the Evening or Vesper Prayer, in memory of the beginning of the day on which God started the Creation of the visible world.
And in commemoration of the events of the six days of creation, the Church adopted the one hundred and third Psalm, which begins "Bless the Lord, O My soul," and which refers clearly and plainly to the creation of man, with symbols clearly indicating the symbolized events.

Similarly in the service of this Prayer, the Holy of Holies symbolizes the interior of paradise; the Temple (where the congregation stands) symbolizes the outside of paradise; the opening of the Royal Door, symbolizes our forefathers within paradise; the closing of the door, symbolizes the closing of the gates of paradise after our forefathers were expelled there from; the Priest...repeating the reverential prayer of the Sunset Psalm, symbolizes the expulsion of our forefathers from paradise. The fact that the Prayer consists solely of beseechings and supplications, with the repetition of the Peace Petitions, devoid of sacrifices, signifies that our forefathers' prayer was likewise, and that the Vesper Prayer is a substitute for the evening sacrifice in the Old Covenant (Ez.9:5) Hence, we chant the Psalm (140), "Lord, I cry unto thee; make haste unto me; give ear unto my voice, when I cry unto thee. Let my prayer be set before thee as incense; and the lifting up of my hands as the evening sacrifice" and the rest of the Vesper Psalms.

Furthermore, the appearance of the Priest, who has been hidden within the throne, and his coming out in the procession (Eisodos), from the northern door of the Holy of Holies, which faces the Altar, refers to the fulfillment of God's promise to our forefathers, of the One who should be born and should bruise the serpent's head, namely the God born in Bethlehem, according to the prophecy of the Prophet Micah, who hath appeared unto the world in the flesh having been hidden in his Divinity from their eyes.

The incense burner in the Priest's hand as he comes out of the sanctuary signifies that God accepted Adam's repentance as a scent of sweet odor, and also the grace of the most Holy Spirit, which was bestowed on Adam and the whole world through the annunciation of the Gospel of the only Son of God. The lighted candle in front of the Priest and his standing in front of the Royal Door and shouting "Wisdom! Let us attend," signify that the flame of that spear with which the angel prevented our forefathers from entering paradise (Gen 3:24) has been quenched by the preaching of the Baptist, who preceded Christ and preached Him, that He was the Light, the Holy Place, and the Entrance into heaven.(John 1), and to the fact that this has been ours through Orthodox faith in the one God of three persons, the Father, Son and Holy Spirit. Hence after the Priest enters to the throne, we chant the Trinitarian evening hymn of thanksgiving, "O Gladsome Light.." which embraces all that have been mentioned heretofore concerning the divine promise of the Savior's coming, his being God's Wisdom and his brilliant Light. In order that we may refer to the faith of the Fathers of the Old Covenant in the coming Savior we may say after the foregoing, "Prokeimenon," which means, that which had been stated before by the Prophets, the prophecies and symbols with which God was preparing the world to receive the Savior. Hence, we say after the Prokeimenon a verse of the Psalms, or recite the Readings on the evenings of festivals.

We also sing hymns, which we call Stichera, which means Hymns preceded by a Stichos, or a corresponding verse of the Psalms. Aposticha means that which follows the Stichos, as the Evening Prayer, also reminds us of the bringing down of the Savior's body and its burial at evening. And in order that we may point to the end of the Old Testament time, and the substitution of the New Testament in its place by the coming of the Savior, we end our Evening Prayer with the prayer of St Simeon the Elder, which is the last of the prophecies about Christ and which comprises both the truth and the symbol; for the symbol is Simeon himself the last of the Old Testament fathers, and the truth is Jesus Christ, who Simeon having borne in his arms and having known by the Spirit that he was the Lord of the living and dead responded with his prayer "Lord now lettest thou thy servant depart in peace.." Whereupon, we respond with "Holy God" and what follows it, as we make the sign of the Cross on our foreheads. Then we sing the Troparion (Hymns) of Absolution and conclude the Prayer.

Excerpts taken from Appendix of Divine Prayers and Services Compiled by Rev. Seraphim Nassar Pages 1060-1065. Antiochian Orthodox Christian Archdiocese of North America: 4th ed. 1993.