On what date did the Soviet Union declare war on Japan shortly before its surrender?

The Soviet Union declared war on Japan on August 8, 1945.

Soviet Foreign Minister Vyacheslav Molotov informed Japan's ambassador in Moscow that the Soviet Union considered itself at war with Japan beginning August 9. The declaration violated the Soviet-Japanese Neutrality Pact, which had still been in force when the announcement was made.

Soviet forces began their invasion of Japanese-held Manchuria shortly after midnight on August 9. This occurred between the atomic bombings of Hiroshima on August 6 and Nagasaki on August 9, adding a major military crisis to Japan's worsening position.

Japan's leaders had hoped the Soviet Union might mediate with the Western Allies. The Soviet declaration ended that hope and is widely considered, alongside the atomic bombings, an important factor in the decision to surrender.