When did the Winter War between Finland and the Soviet Union begin?

The Winter War, also known as the Russo-Finnish War, began on November 30, 1939, when the Soviet Union launched an attack against Finland.123

The conflict was initiated after Finland rejected Soviet demands for territorial concessions on the Karelian Isthmus, the lease of a naval base at Hanko, and the acquisition of several islands in the Gulf of Finland, which the Soviets sought to secure the defense of Leningrad.123