Today, dearest brethren, we celebrate the birthday of those children who were slaughtered, as the Gospel tells us, by that exceedingly cruel king, Herod. Let the earth, therefore, rejoice and the Church exult — she, the fruitful mother of so many heavenly champions and of such glorious virtues. Never, in fact, would that impious tyrant have been able to benefit these children by the sweetest kindness as much as he has done by his hatred. For as today's feast reveals, in the measure with which malice in all its fury was poured out upon the holy children, did heaven's blessing stream down upon them.

"Blessed are you, Bethlehem in the land of Judah! You suffered the inhumanity of King Herod in the murder of your babes and thereby have become worthy to offer to the Lord a pure host of infants. In full right do we celebrate the heavenly birthday of these children whom the world caused to be born unto an eternally blessed life rather than that from their mothers' womb, for they attained the grace of everlasting life before the enjoyment of the present. The precious death of any martyr deserves high praise because of his heroic confession; the death of these children is precious in the sight of God because of the beatitude they gained so quickly. For already at the beginning of their lives they pass on. The end of the present life is for them the beginning of glory. These then, whom Herod's cruelty tore as sucklings from their mothers' bosom, are justly hailed as "infant martyr flowers"; they were the Church's first blossoms, matured by the frost of persecution during the cold winter of unbelief.

http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2013-12-28