The Holy Innocents: that's the name traditional Catholic piety uses to refer to the children, 2 years and under, that Herod killed in Bethlehem, hoping by this to also kill the Christ Child. Eastern iconography portrays the event as "Rachel Weeping for Her Children," a reference to the Prophet Jeremiah (3:15-17) also echoed in St. Matthew's Gospel (2:16-18):
Thus saith the LORD; A voice was heard in Ramah, lamentation, and bitter weeping; Rachel weeping for her children refused to be comforted for her children, because they were not. Thus saith the LORD; Refrain thy voice from weeping, and thine eyes from tears: for thy work shall be rewarded, saith the LORD; and they shall come again from the land of the enemy. And there is hope in thine end, saith the LORD, that thy children shall come again to their own border.
The Troparion of the Feast is very succint; it highlights the apparent contradiction of the tragedy:
As acceptable victims and freshly plucked flowers, as divine firstfruits and newborn lambs, you were offered to Christ who was born as a Child, O most pure children. You mocked Herod's wickedness: now we beseech you, unceasingly pray for our souls. (Source)
Bishop St. Quodvultdeus, in today's Office of Readings, provides another insight into the event:
A tiny child is born, who is a great king. Wise men are led to him from afar. They come to adore one who lies in a manger and yet reigns in heaven and on earth. When they tell of one who is born a king, Herod is disturbed. To save his kingdom he resolves to kill him, though if he would have faith in the child, he himself would reign in peace in this life and for ever in the life to come.Why are you afraid, Herod, when you hear of the birth of a king? He does not come to drive you out, but to conquer the devil. But because you do not understand this you are disturbed and in a rage, and to destroy one child whom you seek, you show your cruelty in the death of so many children.
You are not restrained by the love of weeping mothers or fathers mourning the deaths of their sons, nor by the cries and sobs of the children. You destroy those who are tiny in body because fear is destroying your heart. You imagine that if you accomplish your desire you can prolong your own life, though you are seeking to kill Life himself.
Yet your throne is threatened by the source of grace, so small, yet so great, who is lying in the manger. He is using you, all unaware of it, to work out his own purposes freeing souls from captivity to the devil. He has taken up the sons of the enemy into the ranks of God’s adopted children.
The children die for Christ, though they do not know it. The parents mourn for the death of martyrs. The child makes of those as yet unable to speak fit witnesses to himself. See the kind of kingdom that is his, coming as he did in order to be this kind of king. See how the deliverer is already working deliverance, the saviour already working salvation.
But you, Herod, do not know this and are disturbed and furious. While you vent your fury against the child, you are already paying him homage, and do not know it.
How great a gift of grace is here! To what merits of their own do the children owe this kind of victory? They cannot speak, yet they bear witness to Christ. They cannot use their limbs to engage in battle, yet already they bear off the palm of victory. (Source)
Finally, the antiphon to the Benedictus in today's Morning Prayer is most eloquent:
At the king's command these innocent babies and little children were put to death; they died for Christ, and now in the glory of heaven as they follow him, the sinless Lamb, they sing for ever: Glory to you, O Lord.
It is most appropriate that we also remember today the little babies killed in today's abortion mills. Let us ask their intercession for us, their mothers, and those who defend their deaths as the free exercise of an inalienable "right." May we repent, do penance, and turn to God for forgiveness as doers and enablers of this grievous sin