Edited from a variety of translations (mentioned in the preface) by H. R. Percival
Many learned men now followed Mansi, the greater number blindly; others, again, sought to contradict him, at first the learned Dominican, Mamachi; then Dr. Wetzer (Professor at Freiburg); and latterly, we ourselves in a treatise, "Controversen über die Synode von Sardika," in the Tübinger Theol. Quartalschrift, 1852. Soon after there was a fresh discovery. Some of the Paschal Letters of St. Athanasius, which until then were supposed to be lost, were discovered in an Egyptian monastery, with a very ancient preface translated into Syriac, and were published in that language by Cureton in London, and in the year 1852 in German by Professor Larsow, at the Grey Friars Convent, in Berlin.
Among these Festal Letters, the nineteenth, intended for Easter 347, and therefore composed in the beginning of that year, had been rewritten in Alexandria, as the introduction expressly states. This confirms the statement of the Historia Acephala, that Athanasius was already returned to Alexandria in October, 346, and confirms the chief points of Mansi's hypothesis; while, on the other hand, it unanswerably refutes, by Athanasius' own testimony, the statements of Socrates and Sozomen (which, from their dependence on each other, only count as one), with reference to the date 347.
As we said, Mansi placed this Synod in the year 344; but the old preface to the Festal Letters of St. Athanasius dates it in the year 343, and in fact we can now only hesitate between the dates 343 and 344. If the preface were as ancient and as powerfully convincing as the Festal Letters themselves, then the question concerning the date of the Council of Sardica would be most accurately decided. As, however, this preface contains mistakes in several places, especially chronological errors--for instance, regarding the death of Constantine the Great--we cannot unconditionally accept its statement as to the date 344, but can only do so when it corresponds with other dates concerning that time.
Let us, at all events, assume that Athanasius came to Rome about Easter, 340. As is known, he was there for three whole years, and in the beginning of the fourth year was summoned to the Emperor Constans at Milan. This points to the summer of 343. From thence he went through Gaul to Sardica, and thus it is quite possible that that Synod might have begun in the autumn of 343. It probably lasted, however, until the spring; for when the two envoys, Euphrates of Cologne, and Vincent of Capua, who were sent by the Synod to the Emperor Constans, arrived in Antioch, it was already Easter 344. Stephen, the bishop of the latter city, treated them in a truly diabolical manner; but his wickedness soon became notorious, and a synod was established, which deposed him after Easter 344. Its members were Eusebians, who therefore appointed Leontius Castratus as Stephen's successor, and it is indeed no other than this assembly which Athanasius has in mind, when he says it took place three years after the Synod in Encaeniis, and drew up a very explicit Eusebian confession of faith, the makrostichos.
The disgraceful behaviour of Bishop Stephen of Antioch for some time inclined the Emperor to place less confidence in the Arian party, and to allow Athanasius's exiled clergy to return home in the summer of 344. Ten months later, the pseudo-bishop, Gregory of Alexandria, died (in June, 345), and Constantius did not permit any fresh appointment to the see of Alexandria, but recalled St. Athanasius by three letters, and waited for him more than a year. Thus the see of Alexandria remained unoccupied for more than a year, until the last six months of 346. At length, in October, 346, Athanasius returned to his bishopric.
Reference address : https://www.elpenor.org/ecumenical-councils/sardica-343.asp?pg=2