Okay, so a long time ago, there was a big fight called World War II. Japan was one of the countries that was fighting in it.
After a while, the other countries that were fighting against Japan got together and made a plan to try and end the fighting. They dropped big bombs called atomic bombs on two cities in Japan called Hiroshima and Nagasaki. It was a really big explosion and a lot of people got hurt or died.
After that, Japan realized that they couldn't keep fighting and they decided to surrender. The day when they officially surrendered is called Victory over Japan Day, or V-J Day for short.
People all over the world were really happy because it meant that the war was finally over and no more people would have to get hurt or killed. They celebrated by having big parties and parades in the streets. It was a very important day in history!