In Christianity, this day commemorates the Holy Innocents, the tragedy recorded in the Gospel of Matthew where King Herod orders the killing of all the young male children in and around Bethlehem in order to slay Jesus. Matthew uses the story (which apparently has no historical evidence apart from the gospel account) as a fulfillment of biblical prophecy, and it connects the baby Jesus to the baby Moses, who also escaped a similar massacre. This is also a day of commemoration on Western church calendars; Orthodox churches commemorate the Holy Innocents on December 29th.
This site gives some of the historical information about the day.
Tomorrow (December 29, 2015) is also the 125th anniversary of the massacre at Wounded Knee, another kind of slaughter of innocent persons. http://blog.nmai.si.edu/main/2015/12/125-commemoration-wounded-knee.html