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 republic or a significant date for a patron saint or a ruler. 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. (From: Wikipedia)