1940 (MCMXL, in Roman numerals) was a leap year, of 366 days, of the Gregorian Calendar, its dominical letters were G and F, it had 52 weeks, starting on a Monday, and ending on a Tuesday. .
January 2 - Winter War: USSR launches offensive on Karelian Isthmus, Finland.
February 14 - World War II: The United Kingdom announces that it will arm merchant ships in the North Sea.
February 15 - World War II: The Red Army breaks through the Mannerheim Line of Finnish defense.
March 5 – Six high-ranking members of the Soviet Politburo, including Josef Stalin, sign an order for the execution of 25,700 Polish intelligentsia, including 14,700 Polish prisoners of war, in what will be known as the Katyn Massacre.
March 12 - Winter War: Finland signs an armistice with the USSR.
March 28 - World War II: France and the United Kingdom decide that they will not separately conclude armistices.
April 9 - World War II: Nazi army begins occupation of Denmark and invades Norway.
April 10 - World War II: First naval battle between British and Germans at Narvik, a port in northern Norway.
April 13 - World War II: Second naval battle of Narvik, Norway.
April 14 - World War II: British troops land near Narvik, Norway.
April 19 - World War II: French troops land in Norway.
April 24 - World War II: The French-British army is defeated in Norway at Lillehammer.
May 10 - World War II: Germany invades Belgium, Holland and Luxembourg.
May 10 - World War II: British Prime Minister Neville Chamberlain resigns, and Winston Churchill takes office.
May 14 - World War II: The port city of Rotterdam, Netherlands, is under brutal and devastating attacks, which destroyed the city by the Luftwaffe
May 17 - World War II: German troops occupy Brussels, capital of Belgium.
May 20 - Opening of the Auschwitz I concentration camp in Poland.
May 27 - World War II: King Leopold III of Belgium signs an armistice with Nazi Germany.
May 27 - World War II: Operation Dynamo begins.
June 5 - Getúlio Vargas approves broad military collaboration between Brazil and the United States.
June 10 - Norway capitulates.
June 10 - World War II: Italy declares war on the United Kingdom and France.
June 10 - World War II: Brazil, still neutral, accepts to take charge of Italy's interests with the United Kingdom and colonies, which displeases the British.
June 10 - US President Roosevelt speaks at the University of Virginia pledging to assist the Allies and prepare the United States for war.
June 11 - Getúlio Vargas delivers a speech aboard the Battleship Minas Gerais, which is considered sympathetic to fascism.
June 13 - Getúlio Vargas sends a statement to news agencies and the US government, stating that his speech was neither sympathetic to fascism nor opposed to Roosevelt's June 10 speech.
June 13 - World War II: Paris is declared an open city. An open city is one that in times of war is spared from attacks.
June 14 - World War II: German troops enter Paris.
June 14 - World War II: Brazilian government issues a note reaffirming Brazil's alignment with the United States and American nations, and its neutrality in the war.
June 17 - World War II: General Pétain proclaims himself Head of State of France and asks Nazi Germany for an armistice.
June 21st to June 30th - the II Meeting of