| Millennium: | 1st millennium |
|---|---|
| Centuries: | 8th century – 9th century – 10th century |
| Decades: | 830s 840s 850s – 860s – 870s 880s 890s |
| Years: | 863 864 865 – 866 – 867 868 869 |
| 866 by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishment and disestablishment categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 866 DCCCLXVI |
| Ab urbe condita | 1619 |
| Armenian calendar | 315 ԹՎ ՅԺԵ |
| Assyrian calendar | 5616 |
| Bahá'í calendar | -978–-977 |
| Bengali calendar | 273 |
| Berber calendar | 1816 |
| English Regnal year | N/A |
| Buddhist calendar | 1410 |
| Burmese calendar | 228 |
| Byzantine calendar | 6374–6375 |
| Chinese calendar | 乙酉年十二月十一日 (3502/3562-12-11) — to —
丙戌年十一月廿一日(3503/3563-11-21) |
| Coptic calendar | 582–583 |
| Ethiopian calendar | 858–859 |
| Hebrew calendar | 4626–4627 |
| Hindu calendars | |
| - Vikram Samvat | 922–923 |
| - Shaka Samvat | 788–789 |
| - Kali Yuga | 3967–3968 |
| Holocene calendar | 10866 |
| Iranian calendar | 244–245 |
| Islamic calendar | 251–252 |
| Japanese calendar | |
| Korean calendar | 3199 |
| Minguo calendar | 1046 before ROC 民前1046年 |
| Thai solar calendar | 1409 |
Year 866 (DCCCLXVI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.