Wikipedia
Foundation Day is a designated date on which celebrations mark the founding of a nation, state or a creation of a military unit. This day is for countries that came into existence without the of gaining independence. Older countries that use some other event of special significance as their 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. This holiday can be symbolized by the date of becoming republic or a significant date for a patron saint or a ruler (birthday, accession, removal, etc.) as the starting point of the nation's history. Often the day is not called "Foundation Day" but serves and can be considered as one.