If you have studied the recent history of Japan, you probably know World War II ended for the nation on Aug. 15, 1945, when Emperor Hirohito made his famous radio broadcast announcing Japan's unconditional surrender, as demanded by the Allied powers in the Potsdam Declaration.
Technically, though, you are wrong: Notice of the declaration's acceptance was published in the Kanpo, the official gazette of the Japanese government, during the waning hours of the previous day. Admittedly, the Allies probably weren't subscribers at the time, and the Japanese people didn't know until the next day either, but publication in the official gazette is what makes things real — officially, at least.
Dating back to 1883, the Kanpo is how the government gives official notice of its doings. Published by the National Printing Bureau (which also produces Japan's bank notes, stamps and passports), it was first available only in paper form and delivered to subscribers and outlets around the country. This meant it might take a day or more for hard copies to hit the ground in some parts of the country. However, in a 1958 ruling the Supreme Court case upheld (of course) the sentencing of a defendant to enhanced punishment for narcotics offenses that took effect before the Kanpo announcing the relevant amendment could have been delivered to his location.