Editing 1866 (section)

=== April–June ===
* [[April 4]] &ndash; [[Alexander II of Russia]] narrowly escapes an assassination attempt in the city of [[St Petersburg]].
* [[April 8]] &ndash; The kingdoms of [[Kingdom of Italy|Italy]] and [[Kingdom of Prussia|Prussia]] form an alliance against the [[Austrian Empire]].
* [[April 10]] &ndash; The [[American Society for the Prevention of Cruelty to Animals]] (ASPCA) is founded in New York City by [[Henry Bergh]].
* [[May 1]]&ndash;[[May 3|3]] &ndash; [[Memphis massacre of 1866|The Memphis massacre]]: a rebellion with a series of violent racial events.
* [[May 2]] &ndash; [[Battle of Callao]]: [[Peru]]vian defenders fight the Spanish fleet.
* [[May 7]] &ndash; Student [[Ferdinand Cohen-Blind]] makes a failed attempt to assassinate [[Otto von Bismarck]] in [[Unter den Linden]] in Berlin.
[[File:ShieldNickel.jpeg|thumb|260px|right| [[May 16]]: U.S. [[nickel (U.S. coin)|nickel]] coin approved.]]
* [[May 10]] &ndash; London bank [[Overend, Gurney and Company]] collapses, precipitating [[Panic of 1866]].
* [[May 16]] &ndash; The [[United States Congress]] approves the minting of a nickel 5-cent coin ([[Nickel (United States coin)|nickel]]), eliminating its predecessor, the [[half dime]].
* [[May 24]] &ndash; [[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]] &ndash; First production of the [[comic opera]] ''[[Cox and Box]]'' by [[F. C. Burnand]] and [[Arthur Sullivan]] at Moray Lodge, [[Kensington]]
* [[May 30]] &ndash; [[Bedrich Smetana]]'s comic opera ''[[The Bartered Bride]]'' premiered in [[Prague]].<ref>{{cite encyclopedia|author-link=John Tyrrell (musicologist)|last=Tyrrell|first=John|editor-first=Laura|editor-last=Macy|title=''The Bartered Bride'' (''Prodaná nevěsta'')|encyclopedia=[[Grove Music Online]]}} {{Subscription required}}</ref>
* [[June 2]] &ndash; [[Fenian]] forces skirmish with Canadian militia at the battles of [[Battle of Ridgeway|Ridgeway]] and [[Battle of Fort Erie (1866)|Fort Erie]].
* [[June 5]] &ndash; Calculations indicate [[Pluto]] (not known at this time) reaches its only [[apsis|aphelion]] (furthest point from the [[Sun]]) between [[1618]] and August 2113.
* [[June 8]] &ndash; The [[Parliament of Canada]] meets for the first time in [[Ottawa]].
* [[June 11]] &ndash; The [[Agra High Court]] is established (later shifted to the [[Allahabad High Court]]).
* [[June 14]] &ndash; The [[Austro-Prussian War]] ("Seven Weeks War") begins when the Austrians and most of the medium-size German states declare war on Prussia.
* [[June 20]] &ndash; The Kingdom of Italy declares war on Austria.
* [[June 22]] &ndash; In [[Sweden]], the [[Riksdag of the Estates]] votes to replace itself by an elected two-chamber [[Riksdag]].
* [[June 27]]&ndash;[[June 29|29]] &ndash; [[Battle of Langensalza (1866)|Battle of Langensalza]]: The Prussians defeat the [[Kingdom of Hanover|Hanoverian]] army.