21st day events in history

December 21, 69 The Roman Senate declares Vespasian as Roman emperor, the last in the Year of the Four Emperors
July 21, 230 Pope Pontian succeeds Urban I as the eighteenth pope
November 21, 235 Pope Anterus succeeds Pontian as the nineteenth pope. During the persecutions of emperor Maximinus Thrax he is martyred
July 21, 285 Diocletian appoints Maximian as Caesar and co-ruler
May 21, 293 Roman Emperors Diocletian and Maximian appoint Galerius as Caesar to Diocletian, beginning the period of four rulers known as the Tetrarchy
February 21, 362 Athanasius returns to Alexandria
July 21, 365 A tsunami devastates the city of Alexandria, Egypt. The tsunami was caused by the Crete earthquake estimated to be 8.0 on the Richter scale. Five thousand people perished in Alexandria, and 45,000 more died outside the city
September 21, 455 Emperor Avitus enters Rome with a Gallic army and consolidates his power
June 21, 533 A Byzantine expeditionary fleet under Belisarius sails from Constantinople to attack the Vandals in Africa, via Greece and Sicily
March 21, 537 Siege of Rome: King Vitiges attempts to assault the northern and eastern city walls, but is repulsed at the Praenestine Gate, known as the Vivarium, by the defenders under the Byzantine generals Bessas and Peranius
March 21, 630 Emperor Heraclius returns the True Cross, one of the holiest Christian relics, to Jerusalem
March 21, 717 Battle of Vincy between Charles Martel and Ragenfrid
January 21, 763 The Battle of Bakhamra between Alids and Abbasids near Kufa ends in a decisive Abbasid victory
May 21, 878 Syracuse, Sicily, is captured by the Muslim sultan of Sicily
May 21, 879 Pope John VIII gives blessings to Branimir of Croatia and to the Croatian people, considered to be international recognition of the Croatian state
April 21, 900 The Laguna Copperplate Inscription, the earliest known written document found in the Philippines: the Honourable Namwaran and his children, Lady Angkatan and Bukah, are granted pardon from all their debts by the Commander in chief of Tundun, as represented by the Honourable Jayadewa, Lord Minister of Pailah. Luzon, Philippines
May 21, 996 Sixteen-year-old Otto III is crowned Holy Roman Emperor
November 21, 1009 Lý Công Uẩn is enthroned as emperor of Đại Cồ Việt, founding the Lý dynasty
May 21, 1085 The Swedish town of Helsingborg is founded
April 21, 1092 The Diocese of Pisa is elevated to the rank of metropolitan archdiocese by Pope Urban II
October 21, 1096 People's Crusade: The Turkish army successfully fight off the People's Army of the West
October 21, 1097 First Crusade: Crusaders led by Godfrey of Bouillon, Bohemund of Taranto, and Raymond IV of Toulouse, begin the Siege of Antioch
December 21, 1124 Pope Honorius II is elected
August 21, 1140 Song Dynasty general Yue Fei defeats an army led by Jin Dynasty general Wanyan Wuzhu at the Battle of Yancheng during the Jin–Song wars
December 21, 1140 Conrad III of Germany besieged Weinsberg
March 21, 1152 Annulment of the marriage of King Louis VII of France and Queen Eleanor of Aquitaine
September 21, 1170 Combined English and Irish forces, under the command of Richard de Clare, Earl of Pembroke and Diarmait Mac Murchada, King of Leinster seize Norse-Gaelic Dublin, forcing Ascall mac Ragnaill, King of Dublin into exile
March 21, 1188 Emperor Antoku accedes to the throne of Japan
August 21, 1192 Minamoto no Yoritomo becomes Seii Tai Shōgun and the de facto ruler of Japan. (Traditional Japanese date: July 12, 1192)
October 21, 1209 Otto IV is crowned emperor of the Holy Roman Empire by Pope Innocent III
September 21, 1217 Livonian Crusade: The Estonian leader Lembitu and Livonian leader Kaupo the Accursed are killed in Battle of Matthew's Day
December 21, 1237 The city of Ryazan is sacked by the Mongol army of Batu Khan
July 21, 1242 Battle of Taillebourg : Louis IX of France puts an end to the revolt of his vassals Henry III of England and Hugh X of Lusignan
February 21, 1245 Thomas, the first known Bishop of Finland, is granted resignation after confessing to torture and forgery
June 21, 1307 Külüg Khan is enthroned as Khagan of the Mongols and Wuzong of the Yuan
August 21, 1331 King Stephen Uroš III, after months of anarchy, surrenders to his son and rival Stephen Dušan, who succeeds as King of Serbia
May 21, 1349 Dušan's Code, the constitution of the Serbian Empire, is enacted by Dušan the Mighty
December 21, 1361 The Battle of Linuesa is fought in the context of the Spanish Reconquista between the forces of the Emirate of Granada and the combined army of the Kingdom of Castile and of Jaén resulting in a Castilian victory
November 21, 1386 Timur of Samarkand captures and sacks the Georgian capital of Tbilisi, taking King Bagrat V of Georgia captive
October 21, 1392 Nanboku-chō: Emperor Go-Kameyama abdicates in favor of rival claimant Go-Komatsu
May 21, 1403 Henry III of Castile sends Ruy Gonzáles de Clavijo as ambassador to Timur to discuss the possibility of an alliance between Timur and Castile against the Ottoman Empire
July 21, 1403 Battle of Shrewsbury: King Henry IV of England defeats rebels to the north of the county town of Shropshire, England
March 21, 1413 Henry V becomes King of England
September 21, 1435 An agreement between Charles VII of France and Philip the Good ends the partnership between the English and Burgundy in the Hundred Years' War
February 21, 1440 The Prussian Confederation is formed
May 21, 1502 The island of Saint Helena is discovered by the Portuguese explorer João da Nova
April 21, 1506 The three-day Lisbon Massacre comes to an end with the slaughter of over 1,900 suspected Jews by Portuguese Catholics
April 21, 1509 Henry VIII ascends the throne of England on the death of his father, Henry VII
October 21, 1512 Martin Luther joins the theological faculty of the University of Wittenberg
October 21, 1520 João Álvares Fagundes discovers the islands of Saint Pierre and Miquelon, bestowing them their original name of "Islands of the 11,000 Virgins"