April: the Nazi German extermination camp Sobibor opens in occupied Poland on the outskirts of the town of Sobibór. Between April 1942 and October 1943, at least 160,000 people are killed in the camp.
April 3: Japanese forces begin an all-out assault on the United States and Filipino troops on the Bataan Peninsula.
April 5: the Japanese Navy attacks Colombo in Ceylon (Sri Lanka). Royal Navy Cruisers HMS Cornwall (56) and HMS Dorsetshire (40) are sunk southwest of the island.
April 9: The Bataan Peninsula falls and the Bataan Death March begins. - The Japanese Navy launches an air raid on Trincomalee in Ceylon (Sri Lanka); the Royal Navy aircraft carrier HMS Hermes (95) and Royal Australian Navy destroyer HMAS Vampire (D68) are sunk off the country's East Coast.
April 13: The United States Federal Communications Commission's minimum programming time required of TV stations is cut from 15 hours to 4 hours a week during the war.
April 14: The submarine HMS Upholder (P37) is sunk.
April 14: The German submarine U-85 (1941) is sunk by USS Roper (DD-147).
April 15: King George VI awards the George Cross to Malta to mark the Siege of Malta, saying, "To honour her brave people I award the George Cross to the Island Fortress of Malta, to bear witness to a heroism and a devotion that will long be famous in history (from January 1 to July 24, there is only one 24-hour period during which no bombs fall on this tiny island)."
April 17: Henri Giraud the French commander captured in 1940, escapes from Königstein Fortress.
April 18: Tokyo, Japan, is attacked by the Doolittle Raid, a small force of B-25 Mitchell bomber aircraft commanded by then-Lieutenant Colonel James "Jimmy" Doolittle.
April 23: William Temple enthroned as Archbishop of Canterbury.
April 26: The Reichstag meets for the last time, dissolving itself and proclaiming Adolf Hitler the "Supreme Judge of the German People", granting him the power of life and death over every German citizen.
April 27: A national plebiscite is held in Canada on the issue of conscription.
April 27: The Jewish Star of David is required wearing for all Jews in the Netherlands and Belgium; Jews in other Nazi-controlled countries have already been wearing it.
April 29: An explosion at a chemical factory in Tessenderlo, Belgium leaves 200 dead and 1,000 injured. (source: Wikipedia)