Feria, (Latin for "free day") was a day on which the people, especially the slaves, were not obliged to work, and on which there were no court sessions. In ancient Rome the feriae publicae, legal holidays, were either stativae, recurring regularly (e.g. the Saturnalia), conceptivae, i.e. movable, or imperativae, i.e. appointed for special occasions.
When Christianity spread, on the feriae (feasts) instituted for worship by the Church, the faithful were obliged to attend Mass; such assemblies gradually led, for reasons both of necessity and convenience, to mercantile enterprise and market gatherings which the Germans call Messen, and the English fairs. They were fixed on saints' days (e.g. St. Barr's fair, St. Germanus's fair, St. Wenn's fair, etc.)
In the Roman Rite liturgy, the term feria is used to denote days of the week other than Sunday and Saturday. Various reasons are given for this terminology. The sixth lesson for December 31 in the pre-1971 Roman Breviary says that Pope St. Silvester ordered the continuance of the already existing custom "that the clergy, daily a...