|Covid-19|Spring – Siege of Serdica: Krum, ruler (khan) of the Bulgarian Empire, captures the fortress of Serdica (modern Sofia), after a long siege. According to Byzantine sources, he massacres the garrison (supposedly 6,000 men) and razes the city walls, while returning with much loot to Bulgaria. In the following years (and centuries), Serdica will serve as a base for the expansion of the Bulgars to the south of the Balkans.
A Byzantine fleet lands in the Venetian Lagoon, and attacks a Frankish flotilla at Comacchio, but is defeated. Doge Obelerio degli Antenori marries a Frankish bride, Carola; she becomes the first dogaressa of Venice.
Aznar Galíndez I succeeds Aureolus, as count of Aragon (modern Spain). He is installed by King Louis the Pious (a son of emperor Charlemagne), and remains a Frankish vassal.
A rebellion in Gharb al-Andalus (modern Portugal) is crushed by the Emirate of Córdoba.
March 24 – Caliph Harun al-Rashid dies at Tus, on an expedition to put down an uprising in Khorasan (modern Iran). He is succeeded by his son Muhammad ibn Harun al-Amin.
Emperor Heizei becomes ill, and abdicates the throne in favor of his brother Saga, who is installed as the 52nd emperor of Japan.
Emperor Govinda III defeats his rival Nagabhata II, and obtains the submission of the Pala Empire (India).
Council of Aachen: Frankish bishops adopt the filioque addition in the Creed. Pope Leo III intervenes, and refuses to recognize it as valid.
Hunayn ibn Ishaq, Muslim scholar and physician (d. 873)
Jing Zong, emperor of the Tang Dynasty (d. 827)
Wen Zong, emperor of the Tang Dynasty (d. 840)
March 24 – Harun al-Rashid, Muslim caliph (b. 763)
March 26 – Ludger, Frisian missionary
July 14 – Ōtomo no Otomaro, Japanese general and Shogun (b. 731)
Abbas ibn al-Ahnaf, Muslim poet (b. 750)
Aejang, king of Silla (b. 788)
Aureolus of Aragon, Frankish nobleman
Cellach Tosach mac Donngaile, Irish king
Elfodd, Welsh bishop (approximate date)
Gang, king of Balhae (Korea)
Wang Shizhen, Chinese general (b. 759)
Year 809 (DCCCIX) was a common year starting on Monday (link will display the full calendar) of the Julian calendar.