Saint Nicholas the Mystikos, Patriarch of Constantinople, who upheld the canons against the emperor.
Nicholas was a learned official (mystikos) of the imperial court and disciple of Patriarch Photios before being raised to the patriarchal throne in 901. He resisted the fourth marriage of the Emperor Leo VI, was deposed for it, and later restored.
He worked for peace with Bulgaria and reposed in 925.
Feast day
On the New Calendar, Nicholas Mystikos is commemorated on May 16. Churches on the Old (Julian) Calendar keep the feast on May 29 of the civil calendar.
Also commemorated on May 16

Alexander of Jerusalem
Hierarch, Αλέξανδρος Ιεροσολύμων
Hieromartyr Alexander, Bishop of Jerusalem, disciple of Clement of Alexandria and friend of Origen.
Monastic Martyrs of Saint Sabbas Lavra
Martyr, Αββάδες της Μονής Αγίου Σάββα
Holy Monastic Martyrs of the Lavra of Saint Sabbas, slain by Arab raiders.
Makarios Notaras of Corinth
Hierarch, Μακάριος Νοταράς
Saint Makarios Notaras, Archbishop of Corinth, father of the Kollyvades and compiler of the Philokalia.
Vukasin of Klepci
Martyr, Βουκασίνος
New Martyr Vukasin of Klepci, killed at Jasenovac for refusing to curse Christ.
Name days: Vukasin
Theodore of Vrsac
Hierarch, Θεόδωρος Βρσακ
Hieromartyr Theodore, Bishop of Vrsac, flayed alive by the Turks.