Short answers, pulled from the story.
The Soviet-Japanese War began at one minute past midnight Trans-Baikal time on the 9th of August 1945, when Soviet forces invaded Manchuria simultaneously on three fronts. The campaign concluded on the 2nd of September 1945.
At the Yalta Conference in February 1945, Stalin agreed to enter the Pacific War within two to three months of Germany's defeat in exchange for Allied support for Soviet territorial ambitions in the Far East. Germany surrendered on the 9th of May 1945, setting a deadline of the 9th of August 1945 for the Soviet Union to honor that commitment.
The Soviet Far East Command deployed 89 divisions with 1.5 million men, 3,704 tanks, 1,852 self-propelled guns, 85,819 vehicles, and 3,721 aircraft. Naval forces included 12 major surface combatants, 78 submarines, and the Amur River flotilla.
Historian Tsuyoshi Hasegawa has argued that the Soviet declaration of war was more decisive than the atomic bombings because Japan's home-island defense was designed to repel a southern Allied invasion, leaving no forces to counter a Soviet threat from the north. The Soviet entry also eliminated Japan's hope of using the Soviet Union as a neutral mediator to negotiate a conditional peace. Japan broadcast its surrender message on the 15th of August 1945.
As agreed at Yalta, the Soviet Union annexed South Sakhalin and the Kuril Islands, and gained preeminent interests over Port Arthur and Dalian via the China Changchun Railway. South Sakhalin and the Kuril Islands remain under Russian administration today. Territories on the Asian mainland were transferred to the People's Republic of China in 1955.
The 1st Far East Front was assigned to invade the Korean Peninsula up to the 38th parallel. On the 10th of August 1945, the United States proposed dividing Korean occupation at the 38th parallel north, and the Soviet Union accepted. Soviet forces entered Pyongyang on the 24th of August and established a military government north of that line; American forces landed at Incheon on the 8th of September. This division became the precursor to the Korean War.