May 21 in Eastern Orthodox liturgical calendars is a feast day and a day of commemoration of saints and martyrs. Although some Orthodox churches use the Revised Julian Calendar and others use the traditional Julian calendar, the fixed commemorations for their respective May 21 are broadly the same, no matter which calendar is used.
Holy Equals-to-the-Apostles Emperor Constantine (337), and Empress Helena, his mother (327)
Martyrs Polyeuctus, Victorinus, and Donatus, at Caesarea in Cappadocia.
Hieromartyr Secundus and those with him, in Alexandria (356)
Saint Adelphios, Bishop of Onouphis, in Byzantine Egypt (c. 362)
Saint Christopher I, Patriarch of Antioch (967)
Hieromartyrs Timothy, Polius and Eutychius, deacons, in Mauretania Caesariensis, Morocco.
Hieromartyr Valens and Companions, Bishop who was martyred in Auxerre in France along with three children.
Martyr Secundinus, in Cordoba in Spain under Diocletian (c. 306)
Saint Hospitius the Hermit of Nice, Wonderworker (581)
Saint Barrfoin (Bairrfhionn, Barrindus) of Killbarron (6th century)
Saint Gollen (Collen, Colan) of Denbighshire, a saint who has given his name to Llangollen in Wales (7th century)
Saint Isberga (Itisberga) of Aire, reputed sister of Charlemagne, nun at Aire (Aria) in the Artois, of which she is the patroness (c. 800)
Saint Ageranus (Ayran, Ayrman) of Beze, a monk at Bèze in France, martyred by the Vikings (888)
Saint Theobald of Vienne (Thibaud), Archbishop of Vienne (970-1001)
Right-Believing Sviatoslav II Yaroslavich, Grand Prince of Kiev, son of Saint Yaroslav the Wise (1076) (see also: December 27 - repose)
Blessed Constantine (Yaroslav), prince, and his children Michael and Theodore, Wonderworkers of Murom (1129)
Saint Cyril II (Kirill), Bishop of Rostov (1262)