The Gospels are to be read on the Sabbath [i.e. Saturday], with the other Scriptures.
Ancient Epitome of Canon XVI.
The Gospel, the Epistle [ἀπόστολος ] and the other Scriptures are to be read on the Sabbath.
Before the arrangement of the Ecclesiastical Psalmody was settled, neither the Gospel nor the other Scriptures were accustomed to be read on the Sabbath. But out of regard to the canons which forbade fasting or kneeling on the Sabbath, there were no services, so that there might be as much feasting as possible. This the fathers prohibit, and decree that on the Sabbath the whole ecclesiastical office shall be said.
Neander (Kirchengesch., 2d ed., vol. iij., p. 565 et seq.) suggests in addition to the interpretation just given another, viz.: that it was the custom in many parts of the ancient Church to keep every Saturday as a feast in commemoration of the Creation. Neander also suggests that possibly some Judaizers read on the Sabbath only the Old Testament; he, however, himself remarks that in this case εὐαγγέλια and ἑτέρων γραφῶν would require the article.
Among the Greeks the Sabbath was kept exactly as the Lords day except so far as the cessation of work was concerned, wherefore the Council wishes that, as on Sundays, after the other lessons there should follow the Gospel.
For it is evident that by the intention of the Church the whole Divine Office was designed for the edification and instruction of the people, and especially was this the case on feast days, when the people were apt to be present in large numbers.
Here we may note the origin of our present [Western] discipline, by which on Sundays and feast days the Gospel is wont to be read with the other Scriptures in the canonical hours, while such is not the case on ferial days, or in the order for ferias and “simples.” 177
“Simples” (simplici) are distinguished from “doubles” (duplici) in not having their antiphons said double but only once.