Feast of the Holy Innocents is when the Church remembers the flight of the Holy Family into Egypt and the tragedy of Herod’s orders to kill all the children of Bethlehem that were two years old or younger in the hopes of killing his rival to the throne (See Matthew 2 for the whole story).

Matthew 2:16-28

16 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.

17 Then was fulfilled that which was spoken by Jeremias the prophet, saying: 18 A voice in Rama was heard, lamentation and great mourning; Rachel bewailing her children, and would not be comforted, because they are not.

The New Advent site has this to say about this incident:

The Greek Liturgy asserts that Herod killed 14,000 boys (ton hagion id chiliadon Nepion), the Syrians speak of 64,000, many medieval authors of 144,000, according to Apocalypse 14:3. Modern writers reduce the number considerably, since Bethlehem was a rather small town. Knabenbauer brings it down to fifteen or twenty (Evang. S. Matt., I, 104), Bisping to ten or twelve (Evang. S. Matt.), Kellner to about six (Christus and seine Apostel, Freiburg, 1908); cf. “Anzeiger kath. Geistlichk. Deutschl.”, 15 Febr., 1909, p. 32. This cruel deed of Herod is not mentioned by the Jewish historian Flavius Josephus, although he relates quite a number of atrocities committed by the king during the last years of his reign. The number of these children was so small that this crime appeared insignificant amongst the other misdeeds of Herod.

A modern application: Feast of the Holy Innocents

December 28 is also the feast day of St. Anthony the Hermit.