1588

January–March

• January 22 - Pope Sixtus V issues the papal bull Immensa aeterni Dei, a major reorganization of the Roman Curia creating 15 congregations of cardinals, including the Congregation of the Index Librorum Prohibitorum, the Church list of forbidden books; the Congregation of the Inquisition; and the Congregation of the Vatican Press.
• January 24 - War of the Polish Succession: The Battle of Pitschen takes place at Pitschen with Polish and Lithuanian troops commanded by the Polish hetman Jan Zamoyski defending against an invading Austrian force commanded by Maximilian III, Archduke of Austria. After his army is routed, Archduke Maximilian surrenders and is taken as a prisoner of war, and will be held for more than a year until his release is compelled by the intervention of Pope Sixtus V.
• February 9 - The sudden death of Álvaro de Bazán, 1st Marquis of Santa Cruz, in the midst of preparations for the Spanish Armada, forces King Philip II of Spain to re-allocate the command of the fleet.
• February 18 - In modern-day Sri Lanka, the siege of Colombo by King Rajasinha I of Sitawaka ends when Portuguese Admiral Pedro Teixeira arrives with a fleet of 80 ships and frees the capital of Portuguese Ceylon. King Rajasinha and his troops flee back to his capital at Seethawakapura.
• March 20 - The ascension of Shah Abbas I as Emperor in Iran, of the Safavid Empire, is made official on the first day of the New Year on the Zoroastrian Calendar. Abbas has ruled since October 16, 1587.
• March 25 - The English Army begins the recruitment of volunteers to prepare for the expected invasion by Spain. On April 10, 1593, the English Parliament will enact the first military pension, "An Acte for relief of Soudiours", providing that "forasmuch as it is agreable with Christian Charity Policy and the Honor of our Nation, that shuch as have since the 25th day of March 1588, adventured their lives and lost their limbs or disabled their bodies, or shall hereafter adventure the lives, lose their limbs or disable their bodies, in defence and service of Her Majesty and the State, should at their return be relieved and rewarded to the end that they may reap the fruit of their good deservings, and others may be encouraged to perform like endeavors..."

  April–June

• April 4 - Christian IV becomes king of Denmark–Norway, upon the death of his father, Frederick II.
• May 12 - Day of the Barricades in Paris: Henry I, Duke of Guise seizes the city, forcing King Henry III to flee.
• May 28 - The Spanish Armada, with 130 ships and 30,000 men, begins to set sail from the Tagus estuary, under the command of the Duke of Medina Sedonia and Juan Martínez de Recalde, heading for the English Channel.
• June 18 - Sailing across the South Atlantic Ocean towards England, near the end of their voyage around the world, Thomas Cavendish and his East India Company fleet stumble across the Portuguese-controlled island of Saint Helena. While Saint Helena has been under the control of Portugal for 80 years, England had been unaware of its existence.
• June 19 - Twenty days after departing from Spain, the Spanish Armada receives a foreshadowing of disaster to come during the summer as a storm scatters part of the fleet, postponing the invasion. Alonso de Guzmán y Sotomayor, Duke of Medina Sidonia and commander of the Spanish expedition, returns the fleet to the port of Coruna for repairs, and writes a letter to King Philip, urging him to abandon plans for invasion of England, and to reach an honorable settlement, but the King refuses. The Spanish expedition resumes on July 21.

  July–September

• July 15 - At Rouen, King Henry III of France gives in to the latest demands of the Catholic League and the Duke of Guise, and signs the Edict of Union, agreeing not to allow French Protestants to participate in government, in return for being able to return to Paris.
• July 31 - The first engagement between the English and Spanish fleets results in a victory for the English, under command of Lord Howard of Effingham and Sir Francis Drake.
• August 2 - The English fleet defeats the Spanish fleet, off the Isle of Portland.
• August 7 - The English fleet defeats the Spanish fleet off the coast of Flanders.
• August 8 - Battle of Gravelines: The Spanish Armada is defeated by the English naval force off the coast of Gravelines, in the Spanish Netherlands.
• August 9 - The Duke of Medina Sidonia, commander of the Spanish Armada, decides to return the fleet to Spain after two days of trying to reach the coast of Flanders to meet up with the army of the Duke of Parma.
• August 12 - The fleeing Spanish Armada sails past the Firth of Forth, and the English call off their pursuit, entirely avoiding the storm which destroys most of the Spanish fleet as it attempts to round Scotland and the west coast of Ireland.
• August 19 - Speech to the Troops at Tilbury by Queen Elizabeth I.
• August 29 - In Japan, the Chancellor of the Realm, Toyotomi Hideyoshi, issues an edict for the katanagari, the confiscation of swords from any persons thought to be opposed to his rule.
• September 1 - The Ganja Fortress in modern-day Azerbaijan is captured by Ottoman general Serdar Ferhad Pasha.
• September 9 - English captain Thomas Cavendish completes his circumnavigation in a record time of 781 days, returning to Plymouth more than two years after setting off on July 21, 1586. The previous record had been 1,018 days by the expedition of Sir Francis Drake from 1577 to 1580. By the time of his return, Cavendish has only his flagship, Desire, after having started with two other vessels.
• September 13 - Dutch General Charles III de Croÿ captures the German city of Bonn in the Electorate of Cologne.

  October–December

• October 7 - The first biography of Nicolaus Copernicus is completed by Bernardino Baldi.
• November 13 - Dutch Republic and English forces capture Bergen op Zoom, a fortress in the Spanish Netherlands, after a siege of 41 days.
• November 15 - The English Navy ship Great Spaniard, formerly the Spanish Armada ship San Salvador until its capture on August 1, sinks off of the coast of England's Isle of Purbeck, with the loss of 23 of the 57 crew. The survivors are rescued by an English man-o-war boat.
• December 5 - The Order of Augustinian Recollects is formally recognised as a separate province from the Order of Saint Augustine, an event later known as the Día de la Recolección or Day of Recollection.
• December 23 - Henry III of France strikes his ultra-Catholic enemies, having the Duke of Guise and his brother, Louis II, Cardinal of Guise, killed, and holding the Cardinal de Bourbon a prisoner. As a result, large parts of France reject Henry III as their king, forcing him to side with Henry of Navarre.

  Unknown

• William Morgan's Welsh translation of the Bible is published.
• The Armada Portrait of Elizabeth I of England is created, to celebrate the English defeat of the Spanish Armada, and to assert the strength of Elizabeth herself.