Summary of comments of this office by Dom Schuster, in his work L'année liturgique.

FIRST MASS AT MIDNIGHT

In the Roman rite, it is customary to say three Masses on Christmas; one at first cock crow (ad galli cantum), which became midnight Mass; one at dawn, and one during the day. This tradition is made known to us by St. Gregory, but it is surely more ancient, since the author of the biography of Pope Telesphorus, in the Liber Pontificalis informs us that it was this sovereign pontiff who first introduced the singing of the Gloria in excelsis at the midnight Mass of Christmas.

The Introït is taken from Psalm 2 and can be applied to the divers generations of the word: to His eternal generation in the bosom of the Father; and to His temporal generation, lowly and peaceful, in the virginal womb of Mary; and finally, to that glorious generation in the bowels of the earth, when, on Easter Sunday, He arose to triumph definitively over sin and death.

The Gradual comes from Psalm 109, and first describes in broad lines the eternal day where the Father has begotten, begets, and will always beget the Word, without beginning nor succession of time, without end.

The Alleluia repeats the line of Psalm 2: "The Lord hath said to me: Thou art my Son, this day have I begotten Thee."

The Offertory is taken from Psalm 95, where heaven and earth are invited to rejoice because the Lord has come. For in fact, the coming of Jesus on earth has consecrated the world, as the Church expresses it in her liturgy.

The antiphon for the Communion is taken from Psalm 109, an unquestionably messianic psalm. The Father begot the Word in the splendors of His holiness, so that the tender Child who this day in the crib puts on the livery of the servant and the sinner, is co-eternal and consubstantial with Him.

SECOND MASS AT DAWN

The Introït is taken from the book of the prophet Isaias (Ch. 9). A people that walked in the midst of darkness has today seen a great light. The prophetic antiphon follows, Psalm 92, essentially the psalm of Easter, but it is also well suited to Christmas; for if the Resurrection marks the final triumph of the Savior over death and sin, His birth already announces the dawn of that great day of victory.

The Gradual borrows from Psalm 117 the joyful acclamation of the redeemed to Christ, who makes His first entrance into the world: "Blessed be He that cometh in the name of the Lord!"

The Alleluia is taken from the pascal Psalm 92: "The Lord is clothed with strength, and hath girded Himself with power."

The antiphon of the Offertory is also taken from Psalm 92, which is the psalm of the day. The Child who lies in the crib, is older than the ages. God has given stability to the earth so that it be not shaken, and can serve as a footstool to the throne of the new-born Messias.

The antiphon of the Communion is taken from the book of Zacharias (9:9), who invites the daughters of Sion and Jerusalem to go rejoice in expectation of the coming of Jesus, meek and mild, qualities symbolized by the donkey upon which He is seated in order to take possession of His kingdom.

MASS OF THE DAY

The  Introït is taken from the Book of Isaias (9:6). A child is born to us, and a Son is given to us, who, despite the condition of annihilation to which He has been reduced, is the Eternal, the Creator of the universe, He who by the might of His word, directs and governs everything; on whose shoulder reposes the divine and universal monarchy.

The Gradual is borrowed from Psalm 97. The Lord has manifested to the world the divine Savior, and all the nations have shared in this revelation.

The verse of the Alleluia is drawn from the Byzantine liturgy. This day a sanctified day hath shown upon us. As from all eternity the Father will beget His Word in the bosom of the splendors of His holiness, so today the Blessed Virgin Mary gives birth to the Redeemer who, by His Incarnation, consecrates the world and sanctifies the Church.

The antiphon of the Offertory comes for Psalm 88: "Thine are the heavens, and Thine is the earth, the world and the fullness thereof Thou hast founded: justice and judgment are the preparation of Thy throne."

In the antiphon for the Communion, taken from Psalm 97, we thank the Lord that He has revealed the divine Savior before all nations. Generosity, magnificence, light: such are the characteristics of the works of God.