May 21 (Eastern Orthodox liturgics)

• Holy Equals-to-the-Apostles Emperor Constantine (337), and Empress Helena, his mother (327)^{[1][2][3][note 2]}
• Martyrs Polyeuctus, Victorinus, and Donatus, at Caesarea in Cappadocia.^[5][6]
• Hieromartyr Secundus and those with him, in Alexandria (356)^[2][3][5]
• Saint Adelphios, Bishop of Onouphis, in Byzantine Egypt (c. 362, or c. 431)^{[7][note 3]}
• Saint Boros, monk.^{[8][note 4]}
• Saint Christopher I, Patriarch of Antioch (967)^[9]

• Hieromartyrs Timothy, Polius and Eutychius, deacons, in Mauretania Caesariensis (Morocco).^[5][10][11]
• Hieromartyr Valens and Companions, Bishop who was martyred in Auxerre in France along with three children.^[5][11]
• Martyr Secundinus, in Cordoba in Spain under Diocletian (c. 306)^[5][11]
• Saint Hospitius the Hermit of Nice, Wonderworker (581)^{[2][3][5][11][12]}
• Saint Barrfoin (Bairrfhionn, Barrindus) of Killbarron (6th century)^{[11][note 5]}
• Saint Gollen (Collen, Colan) of Denbighshire, a saint who has given his name to Llangollen in Wales (7th century)^[11]
• Saint Isberga (Itisberga) of Aire, reputed sister of Charlemagne, nun at Aire (Aria) in the Artois, of which she is the patroness (c. 800)^[11]
• Saint Ageranus (Ayran, Ayrman) of Beze, a monk at Bèze in France, martyred by the Vikings (888)^[11]
• Saint Theobald (Thibaud), Archbishop of Vienne (970-1001)^[11]

• Blessed Constantine (Yaroslav), prince, and his children Michael and Theodore, Wonderworkers of Murom (1129)^[2][3][13]
• Saint Cyril II (Kirill), Bishop of Rostov (1262)^[2][14][15]
• Saint Basil of Ryazan, Bishop (1295)^{[2][3][16][note 6]}
• Saint Helen of Decani, Serbia, Princess (c. 1350)^[2][3][17]
• Venerable Cassian the Greek, monk of Uglich, Wonderworker (1504)^{[2][3][18][19]}
• Saint Agapitus of Markushev (Vologda) the Hieromartyr, Abbot (1584)^[2][3][20]
• Saint Constantine the Fool-for-Christ, of Novotorzanin, Russia (c. 16th century)^{[21][note 7]}
• New Martyrs King Constantine Brancoveanu of Wallachia and his four sons Constantine, Stephen, Radu, and Matthew, and his counsellor Ianache (1714)^{[22][note 8]}
• Martyr Pachomios of Patmos (1730)^[3][23]

• Synaxis of Hieromartyrs of Karelia.^[2][3][19]
• Synaxis of Hieromartyrs of Ufa.^[3][19][24]

• Uncovering of the relics (1998) of St. Andrew Ogorodnikov, fool-for-Christ of Simbirsk (1841)^[2]
• Repose of Elena Pavlova of Chepel, Kharkiv (1885)^[2]
• Repose of Elder Isaac of Dionysiou Monastery of Mount Athos (1932)^[2]
• The Anastenaria fire-walking ritual is performed from May 21–23 in some villages in Northern Greece and Southern Bulgaria, coinciding with the feast of Constantine and Helen.^{[25][note 9]}
• Synaxis of all saints of Karelia,^[26]

• Synaxis of the "Vladimir" Icon of the Most Holy Theotokos (1521)^{[2][3][19][27][28][29][note 10]}
• Synaxis of the "Virgin of Tenderness" Icon of the Most Holy Theotokos from Pskov-Pechersk (1524)^{[19][30][31][32][note 11]}

•  
  Sts Constantine and Helena, Holy Equals-to-the-Apostles.
•  
  Sts Constantine and Helena, Holy Equals-to-the-Apostles (Mosaic in Saint Isaac's Cathedral, St. Petersburg, Russia).
•  
  Stained glass window ( 1986 ) showing St Collen (Llangollen, Wales - St.Collen parish church)
•  
  Icon of the Saints of Murom.
•  
  King Constantin Brâncoveanu of Wallachia, and his sons (Church mural in Hurezi Monastery).
•  
  “Vladimir” Icon of the Most Holy Theotokos.
•  
  Mosaic icon of the Virgin of Tenderness, with the eponym "Episkepisis"("The Visitation"). From Triglia in Asia Minor, Church of Saint Basil (late 13th century)

• May 21/June 3. Orthodox Calendar (ORTHOCHRISTIAN.COM).
• June 3 / May 21. HOLY TRINITY RUSSIAN ORTHODOX CHURCH (A parish of the Patriarchate of Moscow).
• Dr. Alexander Roman. May. Calendar of Ukrainian Orthodox Saints (Ukrainian Orthodoxy - Українське Православ'я).
• May 21. Latin Saints of the Orthodox Patriarchate of Rome.
• May 21. The Roman Martyrology.

Greek Sources

• Great Synaxaristes: (in Greek) 21 ΜΑΪΟΥ. ΜΕΓΑΣ ΣΥΝΑΞΑΡΙΣΤΗΣ.
• (in Greek) Συναξαριστής. 21 Μαΐου. ECCLESIA.GR. (H ΕΚΚΛΗΣΙΑ ΤΗΣ ΕΛΛΑΔΟΣ).

Russian Sources

• (in Russian) 3 июня (21 мая). Православная Энциклопедия под редакцией Патриарха Московского и всея Руси Кирилла (электронная версия). (Orthodox Encyclopedia - Pravenc.ru).
• (in Russian) 21 мая (ст.ст.) 3 июня 2013 (нов. ст.). Русская Православная Церковь Отдел внешних церковных связей. (DECR).