A 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 ("fixed," that is, recurring regularly, such as the Saturnalia), conceptivae (movable), or imperativae (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 Bartholomew Fair in London, 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-1962 Roman Breviary says that Pope Sylvester I ordered the continuance of the already existing custom "that the clergy, daily abstaining from earthly cares, would be free to serve God alone". Others believe that the Church simply Christianized a Jewish practice. The Jews frequently counted the days from their Sabbath, and so we find in the Gospels such expressions as una Sabbati and prima Sabbati, the first from the Sabbath.
The early Christians reckoned the days after Easter in this fashion, but, since all the days of Easter week were holy days, they called Easter Monday, not the first day after Easter, but the second feria or feast day; and since every Sunday is the dies Dominica, a lesser Easter day, the custom prevailed to call each Monday a feria secunda, and so on for the rest of the week. The only modern language that fully preserves this Latin ecclesiastical style of naming weekdays is Portuguese, which uses the terms segunda-feira, etc. Greek uses very similar terms, but without the Latin-derived feira. See Week-day names#Numerical for an overview of both systems.
A day on which no saint is celebrated is called a feria (and the celebration is referred to as ferial, the adjectival form of feria). In the present form of the Roman Rite, certain ferias, especially those of Lent, exclude celebration of memorials occurring on the same day, though the prayer of the memorial may be used in place of that of the feria, except on Ash Wednesday and in Holy Week, which exclude even solemnities and feasts.
- Class I: Ash Wednesday and the whole of Holy Week.
- Class II: Advent from 17 December to 23 December and Ember Days.
- Class III: Lent and Passiontide from the day after Ash Wednesday to the day before the Second Sunday in Passiontide, excluding Ember Days.
- Class IV: all other ferias.
In pre-1960 forms of the Roman Rite, ferias were divided into major and minor. The major ferias, which required at least a commemoration even on the highest feast days, were the ferias of Advent and Lent, the Ember days, and the Monday of Rogation week; all others were called minor.
See also 
- This article incorporates information from the Catholic Encyclopedia of 1917.
- Code of Rubrics, 23-26, reproduced in 1962 Roman Missal, p. 21