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866 in various calendars
Gregorian calendar	866; DCCCLXVI
Ab urbe condita	1619
Armenian calendar	315; ԹՎ ՅԺԵ
Assyrian calendar	5616
Balinese saka calendar	787–788
Bengali calendar	272–273
Berber calendar	1816
Buddhist calendar	1410
Burmese calendar	228
Byzantine calendar	6374–6375
Chinese calendar	乙酉年 (Wood Rooster); 3563 or 3356; — to —; 丙戌年 (Fire Dog); 3564 or 3357
Coptic calendar	582–583
Discordian calendar	2032
Ethiopian calendar	858–859
Hebrew calendar	4626–4627
Hindu calendars
- Vikram Samvat	922–923
- Shaka Samvat	787–788
- Kali Yuga	3966–3967
Holocene calendar	10866
Iranian calendar	244–245
Islamic calendar	251–252
Japanese calendar	Jōgan 8; (貞観８年)
Javanese calendar	763–764
Julian calendar	866; DCCCLXVI
Korean calendar	3199
Minguo calendar	1046 before ROC; 民前1046年
Nanakshahi calendar	−602
Seleucid era	1177/1178 AG
Thai solar calendar	1408–1409
Tibetan calendar	阴木鸡年; (female Wood-Rooster); 992 or 611 or −161; — to —; 阳火狗年; (male Fire-Dog); 993 or 612 or −160

Millennium:	1st millennium
Centuries:	8th century; 9th century ; 10th century;
Decades:	840s; 850s; 860s ; 870s; 880s;
Years:	863; 864; 865; 866 ; 867; 868; 869;

Events

April 21 – Bardas, the regent of the Byzantine Empire, is murdered by Basil the Macedonian at Miletus, while conducting a large-scale expedition against the Saracen stronghold of Crete.

May 26 – Basil the Macedonian is crowned co-emperor of the Byzantine Empire, and is adopted by the much younger Michael III.

May 27 – King Ordoño I, ruler of the Kingdom of Asturias, dies after a 16-year reign. He is succeeded by his son, Alfonso III, who later is referred to as "Alfonso the Great".
July 2 – Battle of Brissarthe: Frankish forces, led by Robert the Strong, are defeated by a joint Breton-Viking army.
Louis II, Holy Roman Emperor, defeats the Saracen invaders who are ravaging southern Italy.

The Great Heathen Army of the Vikings rides north to Northumbria. The Northumbrians are preoccupied with a civil war, and the Danes enter York unopposed.^[1]

October 17 – Caliph al-Musta'in is put to death, after a 4-year reign. He is succeeded by al-Mu'tazz, who becomes the youngest Abbasid caliph to assume power.^[2]
The Kharijite revolt against the Abbasid Caliphate begins in Al-Jazira (Upper Mesopotamia), which will last for 30 years.

Fujiwara no Yoshifusa becomes regent (sesshō) to assist the child emperor Seiwa, starting the Fujiwara regency.

Boris I, ruler (knyaz) of the Bulgarian Empire, sends a diplomatic mission, led by the Bulgarian nobleman Peter, to Rome, in an effort to renew ties with the West.
Pope Nicholas I orders that all Catholics should abstain from eating the "flesh, blood, or marrow"^[3] of warm-blooded animals on Wednesdays and Fridays.^[4]
Pope Nicholas I forbids the use of torture in prosecutions for witchcraft (approximate date).

^ John Haywood (1995). Historical Atlas of the Vikings, p. 62. Penguin Books: ISBN 978-0-140-51328-8
^ History of the Arabs by Philip K. Hitti.
^ Dick, Preston (February 24, 2023). "Beyond the Trivia - Fish Fridays". KRCG. Retrieved March 1, 2023.
^ "How Did the Roman Catholic Tradition of Eating Fish on Fridays Begin?". March 15, 2019. Retrieved March 1, 2023.
^ Eustache, D. (1971). "Idrīsids". In Lewis, B.; Ménage, V. L.; Pellat, Ch. & Schacht, J. (eds.). The Encyclopaedia of Islam, Second Edition. Volume III: H–Iram. Leiden: E. J. Brill. pp. 1035–1037. doi:10.1163/1573-3912_islam_SIM_3495. OCLC 495469525.