Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon LXXIII

Canon LXXIII.  (Greek lxxvi.)

The date of Easter and the date of the Council should be announced.

Item, it seemed good that the day of the venerable Easter should be intimated to all by the subscription of formed letters; and that the same should be observed with regard to the date of the Council, according to the decree of the Council of Hippo, that p. 479 is to say the X. Calends of September, and that it should be written to the primates of each province so that when they summon their councils they do not impede this day.

Notes.

Ancient Epitome of Canon LXXIII.

It seemed good that the day of the Holy Easter should be announced on the day of the annual Synod, or on the tenth day before the calends of September.

This is Canon viij. of Carthage, September, a.d. 401.

Johnson.

See Can. 51 (55).

“The time of council,” i.e., of the national council at Carthage.

The Greek canon says ἡ πρὸ δέκα καλανδῶν Σεπτεμβρίων, and Zonaras makes this the 21st of August, but he mistakes in his calculation.