1806 (MDCCCVI) was a common year starting on Wednesday of the Gregorian calendar and a common year starting on Monday of the Julian calendar, the 1806th year of the Common Era (CE) and Anno Domini (AD) designations, the 806th year of the 2nd millennium, the 6th year of the 19th century, and the 7th year of the 1800s decade. As of the start of 1806, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
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- January 1 – The French Republican Calendar is abolished.
- January 1 – The Kingdom of Bavaria is established by Napoleon.
- January 5 – The body of Horatio Nelson, 1st Viscount Nelson, lies in state in the Painted Hall of the Greenwich Hospital, prior to his funeral.
- January 8 – Cape Colony becomes a British colony.
- January 9 – Lord Nelson is given a state funeral at St Paul's Cathedral, attended by the Prince of Wales.
- January 10 – The Dutch in Cape Town surrender to British forces.
- January 19 – The British occupy the Cape of Good Hope.
- January 23 – Grenville succeeds William Pitt the Younger as wartime Prime Minister of the United Kingdom, upon Pitt's death this day amidst worsening health, caused by the stresses of the Napoleonic Wars.
- February 6 – The Royal Navy gains a victory off Santo Domingo.
- March 23 – After traveling through the Louisiana Purchase and reaching the Pacific Ocean, explorers Lewis and Clark and their Corps of Discovery begin their journey home.
- March 28 – Washington College (now Washington & Jefferson College) is chartered by the Pennsylvania General Assembly.
- March 29 – Construction is authorized of the National Road (the first United States federal highway).
- April 8 – Stéphanie de Beauharnais, adopted daughter of Napoleon Bonaparte, marries Prince Karl Ludwig Friedrich of Baden.
- May 30 – Andrew Jackson kills a man in a duel, after the man had accused Jackson's wife of bigamy.
- June 5 – Louis Bonaparte is appointed as King of Holland by his brother, Emperor Napoleon, replacing the Batavian Republic.
- July 4
- July 10 – Vellore Mutiny: Indian sepoys munity against the East India Company, for the first time.
- July 12 – Sixteen German Imperial States leave the Holy Roman Empire and form the Confederation of the Rhine; Liechtenstein is given full sovereignty, leading to the collapse of the Empire after 844 years.
- July 15 – Pike expedition: Near St. Louis, Missouri, United States Army Lieutenant Zebulon Pike leads an expedition from Fort Bellefontaine, to explore the American West.
- July 18 – A gunpowder magazine explosion in Birgu, Malta kills around 200 people.
- July 23 – A British expeditionary force of 1,700 men lands on the left bank of the Río de la Plata, and invades Buenos Aires.
- August 6 – Francis II, the last Holy Roman Emperor, abdicates, thus ending the Holy Roman Empire after about a millennium.
- August 18 – English seal hunter Abraham Bristow discovers the Auckland Islands.  
- September 23 – The Lewis and Clark Expedition reaches St. Louis, Missouri, ending a successful exploration of the Louisiana Territory and the Pacific Northwest. According to one historian, their arrival comes "much to the amazement of residents, who had given the travelers up for dead." 
- September 25 – Prussia issues an ultimatum to Paris, threatening war if France doesn't halt marching its troops through Prussian territory to reach Austria; the message does not reach Napoleon Bonaparte until October 7, and he responds by attacking Prussia. 
- October 8 – Napoleon responds to the September 25 ultimatum from Prussia, and begins the War of the Fourth Coalition; Prussia is joined by Saxony and other minor German states. 
- October 9 – Battle of Schleiz: French and Prussian forces fight for the first time since the war began. The Prussian army is easily defeated, by a more numerous French force.
- October 14 – Battle of Jena–Auerstedt: Napoleon defeats the Prussian army of Prince Hohenlohe at Jena, while Marshal Davout defeats the main Prussian army under Charles William Ferdinand, Duke of Brunswick-Wolfenbüttel, who is killed.
- October 17 – Emperor Jacques I of Haiti (Jean-Jacques Dessalines) is assassinated at the Pont-Rouge, Haiti, and Alexandre Pétion becomes first President of the Republic of Haiti.
- October 24 – French forces enter Berlin.
- October 30 – Capitulation of Stettin: Believing themselves massively outnumbered, the 5,300-man garrison at Stettin in Prussia surrenders to a much smaller French force without a fight.
- November 15 – Pike Expedition: During his second exploratory expedition, Lieutenant Zebulon Pike sees a distant mountain peak while near the Colorado foothills of the Rocky Mountains (later named Pikes Peak in his honor).
- November 21 – Napoleon declares the Continental Blockade against the British, blocking the import of British manufactured goods to the rest of Europe. 
- November 24 – The last major Prussian field force, under Gebhard Leberecht von Blücher, surrenders to the French near Lübeck. Frederick William III has by this time fled to Russia.
- November 28 – French troops enter Warsaw.
- December 26 – War of the Fourth Coalition –
- January 1 – Lionel Kieseritzky, Baltic-German chess player (d. 1853)
- January 27 – Juan Crisóstomo Arriaga, Spanish composer (d. 1826)
- February 22 – Józef Kremer, Polish messianic philosopher (d. 1875)
- March 4
- March 6 – Elizabeth Barrett Browning, English poet (d. 1861)
- March 11 – Carlo Pellion di Persano, Italian admiral, politician (d. 1883)
- March 12 – Jane Pierce, First Lady of the United States (d. 1863)
- March 21 – Benito Juárez, Mexican statesman, folk hero (d. 1872)
- April 3 – Ivan Kireyevsky, Russian literary critic, philosopher (d. 1856)
- April 6 – Friedrich Wilhelm Ritschl, German scholar (d. 1876)
- April 9 – Isambard Kingdom Brunel, British engineer (d. 1859)
- May 2 – Catherine Labouré, French visionary, saint (d. 1876)
- May 4 – William Fothergill Cooke, English inventor (d. 1879)
- May 20 – John Stuart Mill, British philosopher (d. 1873)
- June 12 – John Augustus Roebling, German-American engineer (d. 1869)
- June 27 – Augustus De Morgan, British mathematician, logician (d. 1871)
- July 5 – James Dawson, Aboriginal Guardian (d. 1900)
- July 5 – Blanka Teleki, Hungarian countess, women's rights activist (d. 1862)
- September 12 – Andrew Hull Foote, American admiral (d. 1863)
- October 3 – Oliver Cowdery, American religious leader (d. 1850)
- October 25 – Max Stirner, German philosopher (d. 1856)
- November 13 – Emilia Plater, Polish heroine (d. 1831)
- December 11 – Otto Wilhelm Hermann von Abich, German geologist (d. 1886)
- January 8 – Magdalena Dávalos y Maldonado, Ecuadorian scholar, socialite (b. 1725)
- January 23 – William Pitt the Younger, Prime Minister of the United Kingdom (b. 1759)
- February 2 – Rétif de la Bretonne, French writer (b. 1734)
- February 16 – Franz von Weyrother, Austrian general (b. 1755)
- February 19 – Elizabeth Carter, English writer (b. 1717)
- February 20 – Lachlan McIntosh, Scottish-born American military and political leader (b. 1725)
- March 20 – Salomea Deszner, Polish actress, singer and theater director (b. 1759)
- March 23 – George Pinto English composer (b. 1785)
- March 30 – Georgiana Cavendish, Duchess of Devonshire (b. 1757)
- April 9 – William V, Prince of Orange (b. 1748)
- April 10 – Horatio Gates, retired British soldier who served as an American general during the American Revolutionary War (b. 1727)
- April 22 – Pierre-Charles Villeneuve, French admiral (stabbed) (b. 1763)
- May 9 – Robert Morris (financier), financier of the American Revolution (b. 1734)
- May 24 – John Campbell, 5th Duke of Argyll, British field marshal (b. 1723)
- June 23 – Mathurin Jacques Brisson, French naturalist (b. 1723)
- June 30 – Charles Dickinson, American attorney, famous duelist (killed in a duel by Andrew Jackson) (b. 1780)
- July 4 – Charles Henri Sanson, Royal Executioner of France during the reign of King Louis XVI (b. 1739)
- July 10 – George Stubbs, English painter (b. 1724)
- July 11 – James Smith, American signer of the United States Declaration of Independence
- August 10 – Michael Haydn, Austrian composer (b. 1737)
- August 22 – Jean-Honoré Fragonard, French painter (b. 1742)
- August 23 – Charles-Augustin de Coulomb, French physicist (b. 1736)
- September 9 – William Paterson, signer of the United States Constitution, Governor of New Jersey (b. 1745)
- October 9 – Benjamin Banneker, American astronomer, surveyor (b. 1731)
- October 10 – Louis Ferdinand of Prussia, German prince (killed in battle) (b. 1772)
- October 25 – Henry Knox, Secretary of War under George Washington (b. 1750)
- November 23 – Roger Newdigate, British politician (b. 1719)
- December 22 – William Vernon, American merchant (b. 1719)
- Hibbert, Christopher Nelson: A Personal History (1994) p. 382
- Coleman, Helen Turnbull Waite (1956). Banners in the Wilderness: The Early Years of Washington and Jefferson College. University of Pittsburgh Press. p. 210. OCLC 2191890.
- "Auckland Islands", in Exploring Polar Frontiers: A Historical Encyclopedia, ed. by William J. Mills (ABC-CLIO, 2003) p39
- Jones, A. G. E. (1970). "Captain Abraham Bristow and the Auckland Islands" (PDF). Notes and Queries. 17 (10): 369–371. doi:10.1093/nq/17.10.369. Retrieved 2011-12-14.
- Lee Ann Sandweiss, Seeking St. Louis: Voices from a River City, 1670-2000 (Missouri History Museum, 2000) p41
- F. Loraine Petre, Napoleon's Conquest of Prussia - 1806 (John Lane Company, 1907) pxv
- "Napoleon's Continental Blockade— An Effective Substitute to Naval Weakness?" by Silvia Marzagali, in Naval Blockades and Seapower: Strategies and Counter-Strategies, 1805-2005 ed. by Bruce A. Elleman and S.C.M. Paine (Routledge, 2007) p25
- "Elizabeth Carter - British author". Encyclopedia Britannica. Retrieved 3 January 2017.