National Day is a legal holiday established by a country to commemorate the country itself.

They are usually the country's independence, the signing of the constitution, the birthday of the head of state or other significant anniversaries; some are also the saint's day of the country's patron saint.

Although most countries have similar anniversaries, due to complex political relations, this holiday in some countries cannot be called a National Day. For example, the United States only has Independence Day and no National Day. Both have the same meaning.

In ancient China, the emperor's accession to the throne and birthday were called "National Day".

Today, the National Day of the People's Republic of China refers specifically to October 1, the anniversary of the official founding of the People's Republic of China.

The oldest National Day in the history of the world is the National Day of San Marino. As far back as 301 AD, San Marino designated September 3 as its National Day.