A custom of the 10c and later following the Feast of the Nativity. On the *triduum, or three days, following the Nativity, minor clergy and others were permitted to undertake divine service, with the exception of the mass. On St Stephen's Day (26 December) the deacons took service, followed by priests and choir boys on succeeding days. These occasions were taken in a holiday spirit and often became riotous. They were condemned by the pope in 1207, and in England in 1236; a further prohibition was made in 1390. It was for the far from sober conduct that these occasions were named variously festum fatuorum = feast of fools and asinarium festum = festival of asses or donkeys. Christmas in the Middle Ages was a secular feast as much as it was religious; in the Feast of Fools the two merged.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.