On May 20:
325 The First Council of Nicea – the first Ecumenical Council of the Christian Church was held.
526 An earthquake killed about 300,000 people in Syria and Antiochia.
685 The Battle of Dunnichen or Nechtansmere is fought between a Pictish army under King Bridei III and the invading Northumbrians under King Ecgfrith, who are decisively defeated.
1217 The Second Battle of Lincoln resulting in the defeat of Prince Louis of France by William Marshal, 2nd Earl of Pembroke.
1293 King Sancho IV of Castile created the Study of General Schools of Alcalá.
1497 John Cabot set sail from Bristol,on his ship Matthew looking for a route to the west (other documents give a May 2 date).
1498 Vasco da Gama arrived at Kozhikode (previously known as Calicut), India.
1521 Battle of Pampeluna: Ignatius Loyola was seriously wounded.
1570 Cartographer Abraham Ortelius issued the first modern atlas.
1609 Shakespeare’s Sonnets were first published in London, perhaps illicitly, by the publisher Thomas Thorpe.
1631 The city of Magdeburg in Germany was 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.
1733 Captain James Cook released the first sheep in New Zealand.
1772 Sir William Congreve, English inventor, was born (d. 1828).
1776 Simon Fraser,Canadian Explorer, was born (d.1862).
1799 Honoré de Balzac, French novelist, was born (d. 1850).
1802 By the Law of 20 May 1802, Napoleon Bonaparte reinstated slavery in the French colonies.
1806 John Stuart Mill, English philosopher, was born (d. 1873).
1813 Napoleon Bonaparte led his French troops into the Battle of Bautzen in Saxony, Germany, against the combined armies of Russia and Prussia.
1818 William Fargo, co-founder of Wells, Fargo & Company was born (d. 1881).
1835 Otto was named the first modern king of Greece.
1840 York Minster was badly damaged by fire.
1845 HMS Erebus and HMS Terror with 134 men under John Franklin sailed from the River Thames, beginning a disastrous expedition to find the Northwest Passage.
1861 American Civil War: The state of Kentucky proclaimed its neutrality.
1862 Abraham Lincoln signed the Homestead Act into law.
1864 American Civil War: Battle of Ware Bottom Church – in the Virginia Bermuda Hundred Campaign, 10,000 troops fight in this Confederate victory.
1873 Levi Strauss and Jacob Davis received a U.S. patent for blue jeans with copper rivets.
1882 The Triple Alliance between Germany, Austria-Hungary and Italy was formed.
1883 Krakatoa began to erupt.
1891 The first public display of Thomas Edison’s prototype kinetoscope.
1896 The six ton chandelier of the Palais Garnier fell on the crowd resulting in the death of one and the injury of many others.
1902 Cuba gained independence from the United States. Tomás Estrada Palma became the first President.
1916 The Saturday Evening Post published its first cover with a Norman Rockwell painting (“Boy with Baby Carriage”).
1920 Montreal radio station XWA broadcast the first regularly scheduled radio programming in North America.
1927 By the Treaty of Jedda, the United Kingdom recognizes the sovereignty of King Ibn Saud in the Kingdoms of Hejaz and Nejd, which later merged to become the Kingdom of Saudi Arabia.
1927 At 07:52 Charles Lindbergh took off from Roosevelt Field in Long Island on the world’s first solo non-stop flight across the Atlantic Ocean, touching down at Le Bourget Field in Paris at 22:22 the next day.
1932 Amelia Earhart took off from Newfoundland to begin the world’s first solo nonstop flight across the Atlantic Ocean by a female pilot.
1940 Holocaust: The first prisoners arrived at a new concentration camp at Auschwitz.
1941 New Zealand, British, Australian and Greek forces defending the Mediterranean island of Crete fought desperately to repel a huge airborne assault by German paratroopers.
1946 Cher, American singer, was born.
1949 In the United States, the Armed Forces Security Agency, the predecessor to the National Security Agency, was established.
1949 The Kuomintang regime declared martial law in Taiwan.
1956 In Operation Redwing the first United States airborne hydrogen bomb was dropped over Bikini Atoll in the Pacific Ocean;
1965 PIA Flight 705, a Pakistan International Airlines Boeing 720 – 040 B, crashed while descending to land at Cairo International Airport, killing 119 of the 125 passengers and crew.
1969 The Battle of Hamburger Hill in Vietnam ended.
1980 In a referendum in Quebec, the population rejected by a 60% vote the proposal from its government to move towards independence from Canada.
1983 First publications of the discovery of the HIV virus that causes AIDS in the journal Science by Luc Montagnier and Robert Gallo individually.
1983 A car-bomb explosion killed 17 and injures 197 in the centre of Pretoria.
1985 Radio Martí, part of the Voice of America service, began broadcasting to Cuba.
1989 Chinese authorities declared martial law in the face of pro-democracy demonstrations.
1990 The first post-Communist presidential and parliamentary elections were held in Romania.
1995 In a second referendum in Quebec, the population rejected by a slight majority the proposal from its government to move towards independence from Canada.
1996 The Supreme Court of the United States ruled in Romer v. Evans against a law that would have prevented any city, town or county in the state of Colorado from taking any legislative, executive, or judicial action to protect the rights of gays and lesbians.
2002 Protugal recognised the independence of East Timor , formally ending 23 years of Indonesian rule and 3 years of provisional UN administration (Portugal itself is the former colonizer of East Timor until 1976).
Sourced from Wikipedia & NZ History Online