The feast in question was one of those pre-Counter-reformation celebrations which mixed the sacred and profane in a manner which would put the most 'adventurous' of modern liturgists to shame. It was normally associated with one or other of the church's official feasts which involved the donkey - Christmas, the Flight into Egypt, Palm Sunday, etc... The date and details vary from place to place (mainly in France) but the principal features were a procession in honour of the donkey and a mock-liturgy or liturgies in which the donkey played a central role. (In a footnote Hone notes that traces of the custom could also be found in the treatment of the donkey on Palm Sunday in pre-reformation England. Early in the day one of the priests would prostrate himself before the donkey until 'another priest roused him by the application of a rod of the largest size.' That evening, the choirboys would bargain with the sexton for the use of the doney. They would then take it about the parish singing and collecting money.)
In 11th century Sens the Festa Asinorum took the following form:
On the eve of the day appointed the clergy would process to the Cathedral door and two choristers would sing in high-pitched voices:
Lux hodie, lux laetitiae, me judice, tristisThe ass, vested in precious vestments is then brought into the Cathedral by two canons whilst the hymn Orientis Partibus is sung.
Quisquis erit, revomendus erit, solemnibus istis
Sunt hodie procul invidiae, procul omnia moesta.
Laeta volunt, quicunque colunt asinaria festa.
Poetically translated by Hone:
Light to day, the light of joy - I banish every sorrow;
Wherever found, be it expelled from our solemnities tomorrow.
Away be strife and grief and care, from every anxious breast,
And all be joy and glee in those who keep the Ass's feast.
The Latin lyrics and an English translation can be found here. For the tune click here. (Incidentally, there is a group of musicians based in Assisi called Orientis Partibus. I have their album 'Iubilum' and on it they perform their signature piece, complete with donkey noises!)
The 1st Vespers of the feast was then celebrated, a night-long affir of nonsense chants and parodies of the music of the liturgical year. At intervals wine would be supplied to the particpants when they chanted 'Conductus ad poculum' (brought to the glass). After the 'vespers' the chapter would perform skits in the streets preceded by a huge lantern. When they returned to the Cathedral for the morning offices they would be soaked with pailfuls of water. Between the offices the donkey (still in the Cathedral) would be fed and watered.
At the chant 'Conductus ad ludos' (brought to play) the ass would be led into the nave while people and clergy danced around and brayed like donkeys. After this interlude, the ass was led back into the choir and the 'services' continued. The 2nd vespers would conclude with an invitation to feast ('conductus ad prandium') and the festival concluded with another series of skits in the streets.
One of the variants of the festival recorded by Hone was that of Beauvais. There the 'festa' took place on the 14th of January to commenorate the flight into Egypt. Atop the ass would be placed a handsome girl and a baby (representing the Madonna and child) and these led the procession to the church of St. Stephen. There, the mass would be interupted by the imitation of an ass's bray at various stages and the celebrant would replace the 'Ita Missa est' with a triple 'hee'haw'!