| 54271 | May.11 | Holiday and observance | Vietnam Human Rights Day (Vietnam) |
| 54272 | May.12 | Event | 254 – Pope Stephen I succeeds Pope Lucius I as the 23rd pope. |
| 54273 | May.12 | Event | 304 – Roman Emperor Diocletian orders the beheading of the 14-year-old Pancras of Rome. |
| 54274 | May.12 | Event | 907 – Zhu Wen forces Emperor Ai into abdicating, ending the Tang dynasty after nearly three hundred years of rule. |
| 54275 | May.12 | Event | 922 – After much hardship, Abbasid envoy Ahmad ibn Fadlan arrived in the lands of Volga Bulgars. |
| 54276 | May.12 | Event | 1191 – Richard I of England marries Berengaria of Navarre who is crowned Queen consort of England the same day. |
| 54277 | May.12 | Event | 1328 – Antipope Nicholas V, a claimant to the papacy, is consecrated in Rome by the Bishop of Venice. |
| 54278 | May.12 | Event | 1364 – Jagiellonian University, the oldest university in Poland, is founded in Kraków, Poland. |
| 54279 | May.12 | Event | 1510 – The Prince of Anhua rebellion begins when Zhu Zhifan kills all the officials invited to a banquet and declares his intent on ousting the powerful Ming dynasty eunuch Liu Jin during the reign of the Zhengde Emperor. |
| 54280 | May.12 | Event | 1551 – National University of San Marcos, the oldest university in the Americas, is founded in Lima, Peru. |
| 54281 | May.12 | Event | 1588 – French Wars of Religion: Henry III of France flees Paris after Henry I, Duke of Guise enters the city and a spontaneous uprising occurs. |
| 54282 | May.12 | Event | 1619 – Dutch statesman Johan van Oldenbarnevelt is sentenced to death for high treason. |
| 54283 | May.12 | Event | 1689 – King William's War: William III of England joins the League of Augsburg starting a war with France. |
| 54284 | May.12 | Event | 1743 – Maria Theresa of Austria is crowned Queen of Bohemia after defeating her rival, Charles VII, Holy Roman Emperor. |
| 54285 | May.12 | Event | 1780 – American Revolutionary War: In the largest defeat of the Continental Army, Charleston, South Carolina is taken by British forces. |
| 54286 | May.12 | Event | 1797 – War of the First Coalition: Napoleon I of France conquers Venice. |
| 54287 | May.12 | Event | 1821 – The first major battle of the Greek War of Independence against the Turks is fought in Valtetsi. |
| 54288 | May.12 | Event | 1862 – U.S. federal troops occupy Baton Rouge, Louisiana. |
| 54289 | May.12 | Event | 1863 – American Civil War: Battle of Raymond: Two divisions of James B. McPherson's XVII Corps turn the left wing of Confederate General John C. Pemberton's defensive line on Fourteen Mile Creek, opening up the interior of Mississippi to the Union Army du |
| 54290 | May.12 | Event | 1864 – American Civil War: The Battle of Spotsylvania Court House: Thousands of Union and Confederate soldiers die in "the Bloody Angle". |
| 54291 | May.12 | Event | 1865 – American Civil War: The Battle of Palmito Ranch: The first day of the last major land action to take place during the Civil War, resulting in a Confederate victory. |
| 54292 | May.12 | Event | 1870 – The Manitoba Act is given the Royal Assent, paving the way for Manitoba to become a province of Canada on July 15. |
| 54293 | May.12 | Event | 1873 – Coronation of Oscar II of Sweden |
| 54294 | May.12 | Event | 1881 – In North Africa, Tunisia becomes a French protectorate. |
| 54295 | May.12 | Event | 1885 – North-West Rebellion: The four-day Battle of Batoche, pitting rebel Métis against the Canadian government, comes to an end with a decisive rebel defeat. |
| 54296 | May.12 | Event | 1926 – General Strike: In the United Kingdom, a nine-day general strike by trade unions ends. |
| 54297 | May.12 | Event | 1926 – The Italian-built airship Norge becomes the first vessel to fly over the North Pole. |
| 54298 | May.12 | Event | 1932 – Ten weeks after his abduction, the infant son of Charles Lindbergh, Charles Jr., is found dead in Hopewell, New Jersey, just a few miles from the Lindberghs' home. |
| 54299 | May.12 | Event | 1933 – The Agricultural Adjustment Act is enacted to restrict agricultural production by paying farmers subsidies. |
| 54300 | May.12 | Event | 1935 – Bill Wilson and Dr. Bob Smith (founders of Alcoholics Anonymous) meet for the first time in Akron, Ohio, at the home of Henrietta Siberling. |