That would be August 15, 1945! Japan had announced their surrender on this day, however, they officially surrendered and signed on September 2, 1945. This is the date a 6-year war, finally came to an end. The reason for their surrender, well, the IJN was so devastated that they couldn't keep fighting since Naval Movement is crucial in war. July the fighting had stopped, but it wasn't until August they announced a surrender, and it wasn't until September, that the war ended.
P.S Rip Yamato, Shinagago, and Musashi :(