In the Orthodox Church the Commemoration of the Ordination of Saint John Chrysostom as Archbishop of Constantinople is celebrated on December 15th, while on February 26th is the Commemoration of the Ordination of Saint John Chrysostom as Presbyter. However, according to Socrates the historian, cited below, Chrysostom was ordained Archbishop on February 26th. His ordination as Presbyter therefore should be commemorated on December 15th.
Palladius, Dialogue on the Life of Saint John Chrysostom, Chs. 5 and 6:
"He was ordained deacon by Meletius. By this time his brilliant abilities as a teacher were famous, and the people found in intercourse with him sweet refreshment from the bitterness of life; Bishop Flavianus therefore ordained him presbyter. For twelve years he was a shining light in the Church of Antioch, lending dignity to the priesthood there by the strictness of his life; some he salted with sobriety, some he illuminated by his teaching, some he refreshed with the air of the Spirit. Thus all was fair sailing under the steersmanship of Christ, when the blessed Nectarius, bishop of the Church of Constantinople, fell asleep. Immediately a crowd of people who were not called for rushed forward to secure the supreme position - men who were not men, presbyters by office, yet unworthy of the priesthood; some battering at the doors of officials, others offering bribes, others again going on their knees to the populace. The orthodox laity were much disturbed by all this, and importuned the king with petitions for an experienced priest.
The most influential man of affairs was Eutropius the eunuch, chief of the royal chamberlains. It was his wish to have John in charge of the city, as he had gained some experience of his high character when some business of the king took him to the further East; so he advised the king to send instructions to the governor of Antioch, to send John quietly out of the city, without disturbing the Church. The governor, immediately on receipt of the letter, summoned him to present himself at the shrines of the martyrs, outside the city, near the gate known as Romanesia; where he put him in a public conveyance, and entrusted him to the care of the eunuch sent by Eutropius, and the magistrate's guard. Thus he reached Constantinople, and was ordained bishop of the Church of that city."
Socrates, Ecclesiastical History, Bk. 6, Ch. 2:
"A short time after Nectarius also, bishop of Constantinople died, during the consulate of Cæsarius and Atticus, on the 27th of September. A contest thereupon immediately arose respecting the appointment of a successor, some proposing one person, and some another: at length however it was determined to send for John, a presbyter of the church at Antioch, for there was a report that he was very instructive, and at the same time eloquent. By the general consent therefore of both the clergy and laity, he was summoned very soon afterwards to Constantinople by the Emperor Arcadius: and to render the ordination more authoritative and imposing, several prelates were requested to be present, among whom also was Theophilus bishop of Alexandria. This person did everything he could to detract from John's reputation, being desirous of promoting to that see, Isidore a presbyter of his own church, to whom he was greatly attached, on account of a very delicate and perilous affair which Isidore had undertaken to serve his interests.... The court however gave the preference to John: and inasmuch as many had revived the accusations against Theophilus, and prepared for presentation to the bishops then convened memorials of various charges, Eutropius the chief officer of the imperial bed-chamber collected these documents, and showed them to Theophilus, bidding him 'choose between ordaining John, and undergoing a trial on the charges made against him.' Theophilus terrified at this alternative, consented to ordain John. Accordingly John was invested with the episcopal dignity on the 26th of February, under the following consulate, which the Emperor Honorius celebrated with public games at Rome, and Eutychian, then Praetorian prefect, at Constantinople."
Sozomen, Ecclesiastical History, Bk. 8, Ch. 2:
"Nectarius died about this period, and lengthened debates were held on the ordination of a successor. They all voted for different individuals, and it seemed impossible for all to unite on one, and the time passed heavily. There was, however, at Antioch on the Orontes, a certain presbyter named John, a man of noble birth and of exemplary life, and possessed of such wonderful powers of eloquence and persuasion that he was declared by the sophist, Libanius the Syrian, to surpass all the orators of the age. When this sophist was on his death-bed he was asked by his friends who should take his place. "It would have been John," replied he, "had not the Christians taken him from us." Many of those who heard the discourses of John in the church were thereby excited to the love of virtue and to the reception of his own religious sentiments. For by living a divine life he imparted zeal from his own virtues to his hearers. He produced convictions similar to his own, because he did not enforce them by rhetorical art and strength, but expounded the sacred books with truth and sincerity....
Being, then, held in such high estimation by those who knew him by experience, and by those who were acquainted with him through the reports of others, John was adjudged worthy, in word and in deed, by all the subjects of the Roman Empire, to be the bishop of the church of Constantinople. The clergy and people were unanimous in electing him; their choice was approved by the emperor, who also sent the embassy which should conduct him; and, to confer greater solemnity on his ordination, a council was convened. Not long after the letter of the emperor reached Asterius, the general of the East; he sent to desire John to repair to him, as if he had need of him. On his arrival, he at once made him get into his chariot, and conveyed him with dispatch to a military station, Pagras so-called, where he delivered him to the officers whom the emperor had sent in quest of him. Asterius acted very prudently in sending for John before the citizens of Antioch knew what was about to occur; for they would probably have excited a sedition, and have inflicted injury on others, or subjected themselves to acts of violence, rather than have suffered John to be taken from them.
When John had arrived at Constantinople, and when the priests were assembled together, Theophilus opposed his ordination; and proposed as a candidate in his stead, a presbyter of his church named Isidore, who took charge of strangers and of the poor at Alexandria.... He feared Eutropius, who was artfully eager for this ordination. Eutropius then presided over the imperial house, and they say he threatened Theophilus, that unless he would vote with the other bishops, he would have to defend himself against those who desired to accuse him; for many written accusations against him were at that time before the council."