Massacre of the Innocents. Giotto di Bondone. 1310. Lower Church, San Francesco, Assisi.
Vespers [Greek εσπερινός and Latin vesper, evening] is the traditional evening prayer service. The service is observed before sunset [i.e. at some point between 4 pm and 6 pm] and consecrates the day to God. Vespers is one of the most ancient and solemn of the church offices.
Then Herod, when he saw that he was mocked of the wise men, was exceeding wroth, and sent forth, and slew all the children that were in Bethlehem, and in all the coasts thereof, from two years old and under, according to the time which he had diligently inquired of the wise men.
Matthew 2:16 [KJV]
In The little office of the blessed Virgin Mary, Vespers is associated with the massacre of the innocents. This New Testament story concerns an infanticide ordered by King Herod in response to the reported birth of a child which fulfilled Old Testament prophecies and threatened Herod with the loss of his throne. The slaughter of these infants is considered the first Christian martyrdom.