Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon LXVI

Canon LXVI.

From the holy day of the Resurrection of Christ our God until the next Lord’s day, for a whole week, in the holy churches the faithful ought to be free from labour, rejoicing in Christ with psalms and hymns and spiritual songs; and celebrating the feast, and applying their minds to the reading of the holy Scriptures, and delighting in the Holy Mysteries; for thus shall we be exalted with Christ and together with him be raised up.  Therefore, on the aforesaid days there must not be any horse races or any public spectacle.

Notes.

Ancient Epitome of Canon LXVI.

The faithful shall every one of them go to church during the whole week after Easter.

Van Espen.

It is certain that the whole of Easter week was kept as a feast by the whole Church both East and West; and this Synod did not introduce this custom by its canon, but adopted this canon to ensure its continuance.

Here we have clearly set forth the Christian manner of passing a feast-day, viz., that the faithful on those days did give themselves up to “Psalms and Hymns and Spiritual Songs,” from which the divine office which we call today canonical [i.e., chiefly Mattins and Vespers] are made up; and hence we understand that all the faithful ought to attend the choir-offices, which was indeed observed for many centuries, as I have shewn in my Dissertation on the Canonical Hours, cap. III., § 1, and therefore it was called “public” [or common] prayer.