May 20 is the 140th day of the year (141st in leap years) in the Gregorian calendar; 225 days remain until the end of the year.
685 – The Battle of Dun Nechtain is fought between a Pictish army under King Bridei III and the invading Northumbrians under King Ecgfrith, who are decisively defeated.
794 – While visiting the royal Mercian court at Sutton Walls with a view to marrying princess Ælfthryth, King Æthelberht II of East Anglia is taken captive and beheaded 1217 – The Second Battle of Lincoln is fought near Lincoln, England, resulting in the defeat of Prince Louis of France by William Marshal, 1st Earl of Pembroke.
1497 – John Cabot sets sail from Bristol, England, on his ship Matthew looking for a route to the west 1498 – Portuguese explorer Vasco da Gama discovers the sea route to India when he arrives at Kozhikode (previously known as Calicut), India.
1570 – Cartographer Abraham Ortelius issues Theatrum Orbis Terrarum, the first modern atlas.
1609 – Shakespeare's sonnets are first published in London, perhaps illicitly, by the publisher Thomas Thorpe.
1631 – The city of Magdeburg in Germany is seized by forces of the Holy Roman Empire and most of its inhabitants massacred, in one of the bloodiest incidents of the Thirty Years' War.
1714 – Johann Sebastian Bach leads the first performance of his cantata for Pentecost, Erschallet, ihr Lieder, BWV 172, at the chapel of Schloss Weimar.
1741 – The Battle of Cartagena de Indias ends in a Spanish victory and the British begin withdrawal towards Jamaica with substantial losses.
1775 – The Mecklenburg Declaration of Independence is allegedly signed in Charlotte, North Carolina.
1802 – By the Law of 20 May 1802, Napoleon Bonaparte reinstates slavery in the French colonies, revoking its abolition in the French Revolution.
1813 – Napoleon Bonaparte leads his French troops into the Battle of Bautzen in Saxony, Germany, against the combined armies of Russia and Prussia. The battle ends the next day with a French victory.
1861 – American Civil War: The state of Kentucky proclaims its neutrality, which will last until September 3 when Confederate forces enter the state.
1861 – American Civil War: The State of North Carolina secedes from the Union.
1862 – U.S. President Abraham Lincoln signs the Homestead Act into law, opening eighty-four million acres of public land to white settlers.
1864 – American Civil War: In the Virginia Bermuda Hundred campaign, 10,000 troops fight in this Confederate victory.
1873 – Levi Strauss and Jacob Davis receive a U.S. patent for blue jeans with copper rivets