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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.
- January 1
- January 6 – Ottoman troops clash with supporters of Maronite leader Youssef Bey Karam, at St. Doumit in Lebanon; the Ottomans are defeated.
- January 12
- The Royal Aeronautical Society is formed as The Aeronautical Society of Great Britain in London, the world's oldest such society.
- British auxiliary steamer (1864) sinks in a storm in the Bay of Biscay, on passage from the Thames to Australia, with the loss of 244 people, and only 19 survivors.
- January 18 – Wesley College, Melbourne is established.
- January 26 – Volcanic eruption in the Santorini caldera begins.
- February 7 – Battle of Abtao: A Spanish naval squadron fights a combined Peruvian-Chilean fleet, at the island of Abtao, in the Chiloé Archipelago of central Chile.
- February 13 – The first daylight bank robbery in United States history during peacetime takes place in Liberty, Missouri. This is considered to be the first robbery committed by Jesse James and his gang, although James's role is disputed.
- February 26 – The Calaveras Skull is discovered in California. Purported to be evidence of humans in North America during the Pliocene epoch, it turns out to be a hoax.
- February 28 – The month concludes without having a full moon.
- March 13 – The United States Congress overwhelmingly passes the Civil Rights Act of 1866, the first federal legislation to protect the rights of African-Americans; U.S. President Andrew Johnson vetoes the bill on March 27, and Congress overrides the veto on April 9.
- March 31 – A total lunar eclipse occurs.
- May 10 – London bank Overend, Gurney and Company collapses, precipitating Panic of 1866.
- May 16 – The United States Congress approves the minting of a nickel 5-cent coin (nickel), eliminating its predecessor, the half dime.
- May 24 – Battle of Tuyutí: 32,000 soldiers of the Triple Alliance defeat 24,000 Paraguayan soldiers few miles north of the Paraná, Argentina in the Paraguayan War, with 16,000 casualties.
- May 26 – First production of the comic opera Cox and Box by F. C. Burnand and Arthur Sullivan at Moray Lodge, Kensington
- June 2 – Fenian forces skirmish with Canadian militia at the battles of Ridgeway and Fort Erie.
- June 5 – Calculations indicate Pluto (not known at this time) reaches its only aphelion (furthest point from the Sun) between 1618 and August 2113.
- June 8 – The Parliament of Canada meets for the first time in Ottawa.
- June 11 – The Agra High Court is established (later shifted to the Allahabad High Court).
- June 14 – The Austro-Prussian War begins, when the Austrians and most of the medium German states declare war on Prussia.
- June 20 – The Kingdom of Italy declares war on Austria.
- June 22 – In Sweden, the Riksdag of the Estates votes to replace itself by an elected two-chamber Riksdag.
- June 27–29 – Battle of Langensalza: The Prussians defeat the Hanoverian army.
беларуская (тарашкевіца): 1866
srpskohrvatski / српскохрватски: 1866