Final answer:
The significant event that happened was the end of World War II, marked by Japan's surrender, which led to widespread celebrations and reunions of families in the United States.
Step-by-step explanation:
Based on the provided information, the big event that just happened is the end of a war. The United States celebrated the surrender of Japan with great jubilation. This event marked the end of World War II, leading to the reunification of many families and a significant social and political shift in the postwar period.