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|Centuries:||8th century – 9th century – 10th century|
|Decades:||790s 800s 810s – 820s – 830s 840s 850s|
|Years:||817 818 819 – 820 – 821 822 823|
|820 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1573|
|Bahá'í calendar||-1024 – -1023|
|English Regnal year||N/A|
|Chinese calendar||己亥年 (Earth Pig)
3516 or 3456
— to —
庚子年 (Metal Rat)
3517 or 3457
|- Vikram Samvat||876–877|
|- Shaka Samvat||742–743|
|- Kali Yuga||3921–3922|
|Igbo calendar||-180 – -179|
|Japanese calendar||Kōnin 11
|Minguo calendar||1092 before ROC
|Thai solar calendar||1363|
|Wikimedia Commons has media related to 820.|
- Tahir, the son of a slave, is rewarded with the governorship of Khurasan for supporting the caliphate. He rules independently of Baghdad, not even mentioning the caliph's name in Friday prayers in recognition of caliphal authority.
- Michael II succeeds Leo V as Byzantine Emperor.
- The Historia Brittonum is written (approximate date).
- Fedelmid mac Crimthainn assumes the kingship of Munster.
Science and technology
- Rhodri Mawr ('the Great'), ruler of Gwynedd (Wales) (approximate date) (d. 878)
- Photius I, Patriarch of Constantinople (approximate date) (d. 891)
- December 25 – Leo V, Byzantine Emperor (b. 775) (assassinated)
- Tang Xian Zong, emperor of China (b. 778)
- Adi Shankara, Hindu sage (b. 788).