Cervulus or Cervula is the name of a Roman festival celebrated on the kalends of January (1 January). According to Chambers (1864), remnants seem to have been incorporated into a medieval Christian Feast of the Ass, an opinion endorsed by Carl Jung.
Feast of the Ass
The Feast of the Ass was ostensibly commemorating the role of donkeys in the Flight into Egypt, observed mainly in France. Some accounts of the Christian version of the festival say that a pregnant woman from a village would ride a donkey to a local church. In Beauvais, each operation of the mass, instead of saying "Amen" the congregation would bray "Y-a" There the donkey would stand near the altar during the saying of the mass. Attention to this connection was drawn by Carl Jung, "On the Psychology of the Trickster-Figure" in The archetypes and the collective unconscious (pp. 257–259) Jung mentions "a codex dating apparently from the 11th century", according to which "At the end of the mass, instead of the words 'Ite missa est,' the priest would bray three times. Jung also cites the glossarium of Du Cange for evidence of the medieval custom. Jung (p. 259) postulates "certain tendencies to bring the ass into symbolic relationship with Christ", since "from ancient times, the God of the Jews was vulgarly conceived to be an ass--a prejudice that extended to Christ himself".