On August 15th, 1945, Japan announced its surrender, marking the end of hostilities on the Pacific Theater. However, the official end of World War II came on September 2nd, when Japan signed the act of capitulation. This date is celebrated in the US as the Victory over Japan Day.
From September 1st (12:00 GMT) until September 4th (07:00 GMT), play 3 battles using US vehicles (rank III or higher, with an activity of not less than 70%) to receive the “V-J Day” decal.
Complete the task in random battles, except for Enduring Confrontation missions and [Assault] mode. Track your progress in the hangar by clicking Nickname → Achievements → Holidays → Victory in WWII Day.
You can find the decal in the “Holidays” tab in the Customisation menu.