|Covid-19|Emperor Michael II defeats the rebel forces under Thomas the Slav in Thrace. He and his supporters are forced to seek refuge in Arkadiopolis (modern Turkey). After five months of blockade, Thomas surrenders and is delivered to Michael, seated on a donkey and bound in chains. He pleades for clemency and prostrates before Michael, but is executed.
April 5 – Lothair I, eldest son of Emperor Louis I, is crowned co-emperor again by Pope Paschal I at Rome (initiating the papal practice of handing the imperial sword over, as a symbol of temporal power in the Holy Roman Empire).
King Ceolwulf I of Mercia is deposed by Beornwulf, who takes the throne of Mercia. During his rule he rebuilds the Abbey of St. Peter, and presides over two synods at Clofesho.
May 30 – Emperor Saga abdicates the throne, after a 10-year reign. He is succeeded by his brother Junna, as the 53rd emperor of Japan.
June 13 – Charles the Bald, king of the Franks (d. 877)
Ermentrude of Orléans, queen of the Franks (d. 869)
Muhammad I, Muslim emir of Córdoba (d. 886)
Pepin II (the Younger), king of Aquitaine
Adelochus, archbishop of Strasbourg (b. 786)
Boniface I, margrave of Tuscany
Ceolwulf I, king of Mercia (approximate date)
Gondulphus, bishop of Metz
Han Hong, general of the Tang Dynasty b. 765)
Ljudevit Posavski, duke of Croatia
Thekla, Byzantine empress (approximate date)
Thomas the Slav, Byzantine general and usurper
Timothy I, Syrian patriarch
Wulfheard, bishop of Hereford (approximate date)
Year 823 (DCCCXXIII) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar.