The Forefeast of the Nativity of the Lord begins on December 20. This ancient celebration is testified by St. John Chrysostom in his sermon praising Saint Philogonios, who is also commemorated this day [PG 48:747].
From now on, most of the liturgical hymns will be concerned with the birth of the Savior. Many of the Church's hymns of this period are slightly modified versions of the hymns of Holy Week.
From the 20th to the 23rd we sing the Apolytikion (Tone 4):
"Prepare, O Bethlehem, for Eden has been opened to all. Adorn yourself, O Ephratha, for the Tree of Life blossoms forth from the Virgin in the cave. Her womb is a spiritual paradise planted with the fruit divine; if we eat of it we shall live forever and not die like Adam. Christ is coming to restore the image which He made in the beginning."
We also sing the Kontakion (Tone 3):
"Today the Virgin comes to the cave where she will give birth past understanding to the Word from all eternity. Rejoice, O universe, when the tidings are proclaimed. Glorify, with the angels and shepherds, the one Who chose to be manifest as a newborn Child, while remaining the eternal God."
From the Church hymns for this day: "Let us celebrate, O people, the forefeast of Christ's Nativity; let us raise our minds on high, in spirit going up to Bethlehem". "People, let us celebrate the forefeast of the Nativity of Christ, and let us turn our mind toward Bethlehem, let us raise up thoughts and with spiritual eyes gaze upon the Virgin, who goes into a cave to give birth to the Lord of all". "Let us cast away from us the sleep of idleness, and with wakefulness of soul let us prepare for Christ Who is brought to birth of a pure Maiden". "Rejecting all the defilement of passion, let us take up a prudent disposition worthy of the coming of Christ"; "let us be humbled in spirit, that by good deeds we may exalt Him who is being born".