The Feast of the Holy Innocents dates back to about the fifth century. The massacre of these infants manifests the royal character of Jesus. It is because Herod believed the words of the Magi and those of the High Priests whom he consulted that he sees a rival in the Infant of Bethlehem and jealously pursues Him "that is born King of the Jews." But as the Church sings, in the Hymn for Vespers of the Epiphany, "O cruel Herod, why thus fear, Thy King and God who comes below? No earthly crown comes He to take, who heavenly kingdoms doth bestow."
It is this God-King that the Innocents by dying confess (Collect). Their passion is the exaltation of Christ. And the praise that they render to God is a subject of confusion in the enemies of Jesus (Introit), for, far from attaining their object, they only served to fulfil the saying of the Prophet "out of Egypt have I called my son." (Gospel), and that at Bethlehem would be heard the lamentations of the mothers mourning for their children. To picture their desolation in more vivid colours, Jeremias recalls Rachel whose lamentations are heard in Rama, bewailing her children because they are not. Like a compassionate mother, the Church robes her priests today in vestments of mourning, and suppresses the Gloria and Alleluia.
The Stational mass in Rome is at St Paul-without-the-Walls because the bodies of several of these holy martyrs are venerated there.
Let us show forth in holiness of life, the divinity of Christ, that was confessed by the death of these innocent children.
At that time an angel of the Lord appeared in sleep to Joseph, saying: Arise, and take the child and His mother, and fly into Egypt; and be there until I tell thee: for it will come to pass that Herod will seek the child to destroy Him. Who arose, and took the child and His mother by night, and retired into Egypt; and He was there until the death of Herod: that it might be fulfilled which the Lord spoke by the prophet, saying: Out of Egypt have I called My Son. Then Herod, perceiving that he was deluded by the wise men, was exceeding angry; and sending, killed all the men children that were in Bethlehem, and in all the borders thereof, from two years old and under, according to the time which he had diligently inquired of the wise men. Then was fulfilled that which was spoken by Jeremias the prophet, saying: A voice in Rama was heard, lamentation and great mourning; Rachel bewailing her children, and would not be comforted, because they are not.
(St Matt. 2:13-18)
Hymn from the Breviary
This marvellous hymn, which is sung at Lauds and Vespers of the Holy Innocents, was written by Prudentius in the early 5th century.
Salvéte, flores Mártyrum,
Quos lucis ipso in límine
Christi insecútor sústulit,
Ceu turbo nascéntes rosas.
Vos prima Christi víctima,
Grex immolatórum tener,
Aram sub ipsam símplices
Palma et corónis lúditis.
Jesu, tibi sit glória,
Qui natus es de Vírgine,
Cum Patre et almo Spíritu,
In sempitérna saecula. Amen.
All hail! ye infant martyr flowers,
Cut off in life's first dawning hours:
As rosebuds snapt in tempest strife
When Herod sought your Saviour's life.
You, tender flock of Christ, we sing,
First victims slain for Christ your King:
Beneath the Altar's heavenly ray
With Martyr-palms and crowns ye play.
All honour, laud, and glory be,
O Jesu, Virgin-Born, to thee;
Whom with the Father we adore,
And Holy Ghost, for evermore. Amen.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/07419a.htm