Short answers, pulled from the story.
Soviet foreign minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō that the Soviet Union declared war on Japan at 5 p.m. Moscow time on the 8th of August 1945. The Soviet Government considered itself at war with Japan from the 9th of August.
Marshal Aleksandr Vasilevsky led the Far East Command and devised the plan for the conquest of Manchuria. He reported to Stalin on the 3rd of August that an attack could commence on the morning of the 5th of August if necessary.
The Soviet forces totaled 89 divisions with 1.5 million men, 3,704 tanks, and 1,852 self-propelled guns. There were 85,819 vehicles and 3,721 aircraft available for the operation along with naval forces containing 12 major surface combatants and 78 submarines.
The Khingan, Mukden Offensive Operation ran from the 9th of August to the 2nd of September 1945. This timeframe covered the same dates as the Harbin, Kirin Offensive Operation which included the Battle of Mutanchiang.
The Kwantung Army under General Otozō Yamada was badly trained and poorly equipped with limited supplies despite its large size. Most heavy equipment and best troops had transferred to the Pacific Front over previous three years leaving second-rate units to replace them.