The National Day is a designated date on which celebrations mark the nationhood of a nation or non-sovereign country. This nationhood can be symbolized by the date of independence, of becoming a republic or a significant date for a patron saint or a ruler (birthday, accession, removal, etc.). Often the day is not called "National Day" but serves and can be considered as one. The National Day will often be a national holiday.

　　Some countries have more than one National Day. For example, Pakistan has three National Days, none of which is named the "National Day". This signals the use of a "class" of National Days, that are equally important in the foundation of the nation, and a "class" of less important official public holidays.

　　Most countries have a fixed date National Day, but some have movable dates. An example is Jamaica, which up to 1997 celebrated its National Day on the first Monday in August. This commemorated independence from the United Kingdom which was attained on Monday, 6 August 1962, the first Monday in August of that year. Israel'sIndependence Day is not fixed relative to the civil Gregorian calendar, being formally linked to 5 Iyar on theJewish calendar, but may also be moved to any day between 3 and 6 Iyar in order to avoid preparing for or celebrating either Independence Day or Memorial Day (which immediately precedes Independence Day) on Shabbat; the practical effect of this is that Independence Day may be celebrated any day between 15 April and 15 May. Another example is Thailand which celebrates the birthday of the king on 5 December. This date will change on the accession of the heir to the throne.

　　Most national days can be categorized in two large blocks:

　　Newer countries that celebrate their national day as the day of their independence.

　　Older countries that use some other event of special significance as their national day.

　　Denmark and the United Kingdom are among the few countries that do not have designated national days.