According to the Gospel of Matthew, Herod the Great, alarmed by prophecies of a newborn 'King of the Jews,' ordered the execution of all male children in Bethlehem under age two. The episode, known as the Massacre of the Innocents, has no independent historical corroboration outside scripture but became a fixture of the Christian liturgical calendar, observed on December 28 in the Western church.
Historians have long debated whether the massacre described in Matthew's Gospel reflects an actual event or a theological device linking Jesus's birth to Old Testament prophecy and to Moses's escape from Pharaoh's decree in Exodus. No Roman or Jewish historical source, including the notoriously detailed and unfriendly-to-Herod chronicler Josephus, mentions such a slaughter, which many scholars find telling given Josephus's eagerness to catalog Herod's cruelties.
What's certain is that Herod the Great, who ruled Judea as a Roman client king, was by all contemporary accounts a paranoid and ruthless ruler, responsible for executing several of his own sons and a wife over suspected plots against him. The story of the Innocents fit a ruler whose reputation for violence was already well established.
The feast day became one of profound liturgical significance in the medieval and early modern church, inspiring artworks by Rubens, Bruegel, and Poussin, and giving rise to folk traditions across Europe treating December 28 as an unlucky day, on which no important business should be started.