21. Nomine feriae intelleguntur singuli dies hebdomadae, praeter dominicam.
The term 'feria' means the different days of the week apart from Sunday.
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. ...
When Christianity spread, on the feriae (feasts) instituted for worship by the Church, the faithful were obliged to attend Mass; such assemblies gradually led ... to mercantile enterprise and market gatherings .... the English fairs. ...
In the Roman Rite liturgy, the term feria is used in Latin to denote days of the week other than Sunday and Saturday. ... Various reasons are given for the Latin terminology. ... 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 [of Easter]; and since every Sunday is the dies Dominica, a lesser Easter day, the custom prevailed to call each Monday a feria secunda [second feast day], and so on for the rest of the week. ...
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). ...
The takeaway is that "feria II" does not at all imply that Sunday ("Dominica") is somehow "Feria I", but that "feria II" refers to the (second) day of the week, the complete week being:
Dominica, feria secunda, feria tertia, feria quarta, feria quinta, feria sexta, sabbatum
515. The solemn tone is used in the chant of the collects, the preface, and the Lord's Prayer:
a) on Sundays;
b) in festive Masses and in the Mass of the Saturday office of the Blessed Virgin Mary;
c) on the vigils of the 1st class;
d) on Thursday of the Lord's Supper and in the Mass of the Easter vigil;
e) throughout octaves;
f) in votive Masses of the 1st, 2nd and 3rd class.
516. The ferial tone is used:
a) on ferias;
b) on vigils of the 2nd and 3rd class;
c) in votive Masses of the 4th class;
d) in Masses of the dead.
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