festival celebrated in the Christian churches in the West on December 28 and in the Eastern churches on December 29 and commemorating the massacre of the children by King Herod in his attempt to kill the infant Jesus (Jesus Christ) (Matthew 2:16–18). These children were regarded by the early church as the first martyrs (martyr), but it is uncertain when the day was first kept as a saint's day. At first it may have been celebrated with Epiphany, but by the 5th century it was kept as a separate festival. In Rome it was a day of fasting and mourning.

It was one of a series of days known as the Feast of Fools (Fools, Feast of), and the last day of authority for boy bishops (boy bishop). Parents temporarily abdicated authority. In convents and monasteries the youngest nun and monk were allowed to act as abbess and abbot for the day. These customs, which mocked religion, were condemned by the Council of Basel (Basel, Council of) (1431).

In medieval England the children were reminded of the mournfulness of the day by being whipped in bed in the morning; this custom survived into the 17th century.

The day is still observed as a feast day and, in Roman Catholic countries, as a day of merrymaking for children.