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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.
- 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 Canadian Parliament 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