December 25, Emperor Leo V ("the Armenian") is assassinated by conspirators in the Hagia Sophia, at Constantinople. Though unarmed, he fights back fiercely but dies of his wounds. He is succeeded by Michael II, commander of the palace guard (excubitores). Leo's family (including his mother and his wife Theodosia) is exiled to monasteries in Princes' Islands.
Fedelmid mac Crimthainn assumes the kingship as ruler of Munster (modern Ireland).
Emperor Xian Zong dies from poisoning (due to medicines), after a 14-year reign. He is succeeded by his son Mu Zong, as ruler of the Tang Dynasty of China.
Adalbert I, Frankish margrave (approximate date)
Adelaide of Tours, Frankish noblewoman (approximate date)
Álmos, military leader (gyula) of the Hungarians (approximate date)
Anandavardhana, Indian philosopher (d. 890)
Ashot I ("the Great"), king of Armenia (approximate date)
Buhturi, Syrian poet (d. 897)
Godfrid Haraldsson, Danish Viking king (approximate date)
Grimbald, Frankish Benedictine monk (d. 901)
Hucbert, Frankish nobleman (d. 864)
Qusta ibn Luqa, Syrian Melkite physician (d. 912)
Ranulf I of Aquitaine, Frankish nobleman (d. 866)
Rhodri the Great, king of Gwynedd (Wales) (approximate date)
September 14, Li Yong, chancellor of the Tang Dynasty
December 25, Leo V, emperor of the Byzantine Empire (b. 775)
Adi Shankara, Indian philosopher and theologian (b. 788)
Causantín mac Fergusa, king of the Picts (approximate date)
Huangfu Bo, chancellor of the Tang Dynasty
Lupo III, duke of Gascony (approximate date)
Muhammad ibn Idris al-Shafi‘i, Muslim imam (b. 767)
Olcobhar mac Cummuscach, abbot of Clonfert
Tnúthgal mac Donngaile, king of Munster
Tutu Chengcui, eunuch and advisor of the Tang Dynasty
Wang Chengzong, general of the Tang Dynasty
Xian Zong, emperor of the Tang Dynasty (b. 778)
Year 820 (DCCCXX) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar.