Short answers, pulled from the story.
The Soviet Union and the Empire of Japan signed the non-aggression pact on the 13th of April 1941 in Moscow. This agreement arrived two years after the conclusion of the Soviet-Japanese Border War.
Foreign Minister Vyacheslav Molotov represented the Soviet Union during the signing ceremony in Moscow. His counterparts were Foreign Minister Yosuke Matsuoka and Ambassador Yoshitsugu Tatekawa for Japan.
The text stated that the pact remained valid for five years until the 13th of April 1946. A clause existed stating that if neither party denounced the pact one year before expiration, it would automatically prolong for another five years.
Tokyo chose to invade European colonies in Southeast Asia rather than attack Siberia. The decision allowed Japan to focus its resources elsewhere while maintaining neutrality under the existing pact.
On the 5th of April 1945, the Soviet Union denounced the pact with Japan through official channels. Foreign Minister Molotov informed the Japanese government that the treaty would go out of effect immediately.