Today is the official memorial day for China's victory in the War of Resistance against Japanese Aggression. Now, let's take a look at why September 3rd was chosen and made a national holiday.
今天（9月3日）是中国人民抗日战争胜利的官方纪念日。现在我们一起来看看为何将9月3日定为全国性的纪念日。

Japan's announcement of surrender actually happened on August 15th, 1945. A little after noon, Japanese emperor Hirohito announced his country's acceptance of the terms of the Potsdam Declaration, and its unconditional surrender. The message was broadcast across the nation over radio.
1945年8月15日，日本宣布投降。当天中午时分，日本裕仁天皇宣布日本接受《波茨坦公告》，无条件投降。这一消息通过电台广播传遍全国。

However, without the formal signing over the terms of the surrender, it didn't have any legal standing. So, on September 2nd, representatives from Japan, China and eight other Allied powers gathered on the deck of the USS "Missouri" in Tokyo Bay. They signed on the Japanese instrument of surrender. The ceremony over 20 minutes, and it was broadcast throughout the world.
不过，投降协议尚未正式签署，日本的投降尚不具备法律效力。因此，9月2日，在停泊于东京湾的美国“密苏里号”战列舰上，日本代表同中国及其他八个同盟国的代表签署了日本降伏文书。受降仪式时长20多分钟，并以广播形式昭告世界。

The next day, September 3rd, the Japanese army in China formally surrendered to the Chinese government. The whole of China was in jubilation. Celebrations were held across the country. It marked the first outright victory over a foreign invasion in Chinese modern history. Therefore, September 3rd was chosen, at the time, as the official victory day.
次日，也就是9月3日，中国境内的日本军队正式向中国政府投降。全国人民都沉浸在喜悦之中，举国欢庆。这是近代以来中华民族反抗外敌入侵第一次取得的完全胜利。因此，当时，9月3日就被定为抗战胜利纪念日。

However, after the founding of New China in 1949, the victory day was reset to August 15th, the date Japan announced unconditional surrender. But two years later, the then-Government Administrative Council switched it back to September 3rd.
然而，1949年新中国成立后，胜利日一度被改为8月15日，即日本宣布无条件投降的日期。两年后（1951年），当时的中央人民政府政务院重新将9月3日确定为抗战胜利纪念日。

In 2014, China's top legislative body ratified September 3rd as the "Victory Day of Chinese People's War of Resistance Against Japanese Aggression". It then officially became a national holiday when it was announced that memorial activities would be held every year going forward. It's hoped the ratification will help people reflect on the war; commemorate the heroic sacrifices; and express China's firm stance of safeguarding national sovereignty, territorial solidarity and world peace.
2014年，全国人大通过决议，将9月3日确定为“中国人民抗日战争胜利纪念日”。有关部门宣布，今后每年9月3日都将举行纪念活动，9月3日也正式成为全国性的纪念日。批准设立抗战胜利纪念日有望帮助人们反思战争，缅怀先烈的牺牲，表达中国坚决维护国家主权、国土完整和世界和平的决心。

Vocabulary
USS: 是United States Steamship的缩写，指美国船舶。