When did Japan surrender?

The surrender was on August 15th 1945, formally ratified on September 2nd and was the result of American nuclear attacks on Japan.
In 1945 President Truman decided that due to the suicidal nature of Japanese defences of Pacific islands in the face of American attack, an invasion of the Japanese mainland would be too costly in terms of American losses.
He therefore gave the go ahead to drop a nuclear bomb, Little Boy, on Hiroshima on August 6th 1945. Three days later Fat Man was dropped on Nagasaki and the Japanese subsequently surrendered 6 days later.