In the 1945 V-J Day abbreviation, the letter J stands for Japan.
V-J Day means “Victory over Japan Day,” the name used for celebrations marking Japan’s defeat and surrender in World War II. It parallels V-E Day, or Victory in Europe Day, which marked Nazi Germany’s surrender in May 1945.
The label can refer to more than one date because news crossed international time zones. Japan announced its acceptance of Allied surrender terms on August 15, 1945, while the announcement reached the United States and much of the Americas on August 14. The formal surrender ceremony took place later, on September 2, aboard USS Missouri in Tokyo Bay.
A common mix-up is treating V-J Day as only the signing date. In historical usage, it may describe the public announcement, the celebrations that followed, or the formal end-of-war ceremony, depending on the country and context.