1866 (MDCCCLXVI) was a common year starting on Monday of the Gregorian calendar and a common year starting on Saturday of the Julian calendar, the 1866th year of the Common Era (CE) and Anno Domini (AD) designations, the 866th year of the 2nd millennium, the 66th year of the 19th century, and the 7th year of the 1860s decade. As of the start of 1866, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.

Millennium: 2nd millennium
Centuries: 18th century19th century20th century
Decades: 1830s  1840s  1850s  – 1860s –  1870s  1880s  1890s
Years: 1863 1864 186518661867 1868 1869
1866 in other calendars
Gregorian calendar1866
Ab urbe condita2619
Armenian calendar1315
Assyrian calendar6616
Bahá'í calendar22–23
Balinese saka calendar1787–1788
Bengali calendar1273
Berber calendar2816
British Regnal year29 Vict. 1 – 30 Vict. 1
Buddhist calendar2410
Burmese calendar1228
Byzantine calendar7374–7375
Chinese calendar乙丑(Wood Ox)
4562 or 4502
    — to —
丙寅年 (Fire Tiger)
4563 or 4503
Coptic calendar1582–1583
Discordian calendar3032
Ethiopian calendar1858–1859
Hebrew calendar5626–5627
Hindu calendars
 - Vikram Samvat1922–1923
 - Shaka Samvat1787–1788
 - Kali Yuga4966–4967
Holocene calendar11866
Igbo calendar866–867
Iranian calendar1244–1245
Islamic calendar1282–1283
Japanese calendarKeiō 2
Javanese calendar1794–1795
Julian calendarGregorian minus 12 days
Korean calendar4199
Minguo calendar46 before ROC
Nanakshahi calendar398
Thai solar calendar2408–2409
Tibetan calendar阴木牛年
(female Wood-Ox)
1992 or 1611 or 839
    — to —
(male Fire-Tiger)
1993 or 1612 or 840