The Feast of the Ass
Following Jesus’ birth, Herod ordered the slaying of all male children in the vicinity of Bethlehem who were under two years old, known as the Massacre of the Innocents. The Magi, who had asked Herod about the new king born to the world and caused his paranoia that led to this order, warned Mary and Joseph when they visited on the Epiphany. Joseph and Mary fled to Egypt with the baby Jesus. According to tradition, the Apocrypha, and various paintings of the scene, this journey was undertaken on the back of a donkey. The feast was a celebration of that escape, held on or around 14 January during the Middle Ages.
The custom held that a young girl, representing Mary, would ride a donkey through the town where it would be led into the church. A priest would then hold mass, with the donkey stood beside the altar throughout.
Mary dismounted from her beast, and sat down with the child Jesus in her bosom. And there were with Joseph three boys, and with Mary a girl, going on the journey along with them. And, lo, suddenly there came forth from the cave many dragons; and when the children saw them, they cried out in great terror. Then Jesus went down from the bosom of His mother, and stood on His feet before the dragons; and they adored Jesus, and thereafter retired.The Infancy Gospel of Matthew 18:9
Part of the service involved a play being performed where the priest would argue the divinity of Christ while a false idol stood in the middle of the church which the other players, who would be performing as Biblical prophets, would refuse to bow down to. All this would be overseen by the donkey, who would be blessed for saving Christ. At the end of mass, instead of a parting statement, the priest would bray three times to the people, and they in turn would bray back, as if they were all donkeys.
The tradition was nowhere near as controversial as the Feast of Fools, an annual celebration of chaos held on the first day of the new year, yet it died out around the same time. Although the donkey was venerated, it was still being brought into a church, and therefore was not in keeping with the reverence of the puritanical ideals of late-medieval Christianity.