Holy innocents is a term that refers to the children talked about in Matthew, Chapter 2, verses 16-18.
When Herod realized that he had been outwitted by the Magi, he was furious, and he gave orders to kill all the boys in Bethlehem and its vicinity who were two years old and under, in accordance with the time he had learned from the Magi. Then what was said through the prophet Jeremiah was fulfilled.
"A voice is heard in Ramah, weeping and great mourning, Rachel weeping for her children and refusing to be comforted, because they are no more." (Quoted from Jeremiah 31:15)
The slaughter of the innocents is unattested in secular records, but the historical plausibility of this event happening is consistent with the character and actions of Herod the Great. Besides killing his enemies, he had no qualms in killing family members and friends as well.In other words, absence of evidence is not evidence of absence, and people who deny the historicity of the massacre of the innocents on that basis commit the logical fallacy known as argument from silence.