Today was known in the Middle Ages as the “Feast of the Ass” -- known in France, where it was most popular, as “Fête de l'âne.” It was meant to honor the beast of burden that carried the Blessed Virgin Mary as she and Joseph fled for Egypt with the Infant Jesus. Apparently, at some point in the Mass of the Day, after the Introit (Entrance Chant), the following ditty was sung, in Latin and in French:
Pulcher et fortissimus
Hez, Sire Asnes, car chantez,
Belle bouche rechignez,
Vous aurez du foin assez
Et de l'avoine a plantez.
The folks at Southern Fried Catholism explain it further. As they (apparently) explain, liturgical abuses have a long and colorful history. What they don't explain, is that the chant sung here was later the basis for an unrelated Marian hymn, Concordi Laetitia.
It just goes to show you: Fact is stranger than truth.