Christian feast day: Mar Nestorius (in the Nestorian churches)

Mar, also Mor in Western Syriac, is an Aramaic word meaning "my lord". The corresponding feminine forms in Syriac are Mart and Mort for "my lady".

Nestorius of Constantinople was an early Christian prelate who served as Archbishop of Constantinople from 10 April 428 to 11 July 431. He was a Christian theologian from the Catechetical School of Antioch, and several of his teachings in the fields of Christology and Mariology were seen as controversial and heretical, causing major disputes. In 431, he was condemned and deposed from his see by the Council of Ephesus, presided over by his archrival Cyril of Alexandria, but the counter-council led by John I of Antioch vindicated him and deposed Cyril in return. Nestorius refrained from attending both of these councils and instead sought retirement from the Byzantine Emperor.

The Church of the East or the East Syriac Church, also called the Church of Seleucia-Ctesiphon, the Persian Church, the Assyrian Church, the Babylonian Church, the Chaldean Church or the Nestorian Church, is one of three major branches of Eastern Nicene Christianity that arose from the Christological controversies in the 5th century and the 6th century, alongside that of Miaphysitism and Chalcedonian Christianity.