V-J Day is the abbreviation for “Victory over Japan Day,” marking Japan's defeat in World War II. The term was modeled on V-E Day, which commemorated victory in Europe after Germany's surrender.
The label can refer to more than one date because news of Japan's acceptance crossed the International Date Line. Japan announced the decision on August 15, 1945, while the United States received and reported the news on August 14. September 2 is also associated with V-J Day because the surrender document was signed that day.
This variation explains why different countries observe the end of the Pacific War on different dates. In Japan, August 15 is commonly associated with the end of the war, while the United States commonly commemorates September 2 as the formal end. V-J Day should not be confused with V-E Day, which concerns the European theater and Nazi Germany's surrender in May 1945.