The crimson red color of Christmas betokens the many martyrs venerated by the Church during the octave, who chose to lay down their lives for Christ. These martyrs include St. Stephen, whose feast is celebrated December 26 and the Holy Innocents, whose feast is celebrated on December 28.
Even though St. Stephen is called protomartyr, the Holy Innocents willingly died for Christ many years earlier as a stark reminder of why Christ's coming was so necessary. Venerable Mary of Agreda, in her four-volume work on the life of Christ, Mystical City of God, recounts how Christ gave these infants a special grace to understand and accept their impending martyrdom.
On page 674 of volume two, Incarnation, she writes of Christ's prayer to His Heavenly Father on behalf of these little lambs, who were about to be slaughtered at Herod's command:
He offered up the murdered children as the first fruits of His own Death; asking Him also that they receive the use of reason, in order that they might be a willing sacrifice for their Redeemer and accept their death for His glory. Thus, He would be able to reward them with the crowns of martyrdom for what they suffered. All this, the eternal Father granted.
In Matthew 2:16 the horrific event is recounted: "Then Herod, seeing that he had been tricked by the Magi, was exceedingly angry; and he sent and slew all the boys in Bethlehem and all its neighborhood who were two years old or under, according to the time that he had carefully ascertained from the Magi." In a fourth century homily St. Augustine says the Holy Innocents "died for Christ and in His place."
In the next verse, St. Matthew recalls how this event was actually prophesied. He was referring to the prophet Jeremiah, who in chapter 31:15–17 not only foretold the event but also the fact that those slaughtered would be rewarded:
Thus saith the Lord: Let thy voice cease from weeping, and thy eyes from tears: for there is a reward for thy work, saith the Lord: and they shall return out of the land of the enemy. And here is hope for thy last end, saith the Lord: and the children shall return to their own borders.
Watch the panel discuss the first Christian martyrs in The Download—Holy Innocents.