This holiday is always celebrated on 2 September.

The day marks Vietnam's declaration of independence from France. It is the national holiday of Vietnam

Vietnam National Day History:

The Japanese occupied Vietnam during World War II but allowed the French to remain and exert some influence. At the war's end in August 1945, a power vacuum was created in Vietnam. Capitalizing on this, the Việt Minh launched the "August Revolution" across the country to seize government offices. Emperor Bảo Đại abdicated on August 25, 1945, ending the Nguyễn Dynasty. On September 2, 1945, at Ba Đình Square, Hanoi, leader of the communist Viet Minh organization, declared Vietnam's independence under the new name of the Democratic Republic of Vietnam (DRVN) in a speech that invoked the United States Declaration of Independence and the French Revolution's Declaration of the Rights of Man and of the Citizen

How is Vietnamese National Day celebrated?
To celebrate Vietnam's national day, there are speeches, parades, fireworks, and other festivities across the country, with a large march in Ba Dinh Square in Hanoi, where the declaration of Independence was made.

This is a very patriotic holiday with the national flag of Vietnam displayed everywhere and large posters of Ho Chi Minh ('Uncle Ho') adorning city walls..

September 2nd is truly a special and important day for the whole Vietnam nation.