The day a state marks its own founding — which is not always a day off.
A national day marks a founding, an independence, a unification or a monarch. Almost every state has one and they are among the most reliably fixed dates in the corpus, because they are usually named in a constitution rather than an annual decree.
What varies is what happens. Some national days close everything; some close the public sector; some are ceremonial and close nothing at all. The date is stable and the operational effect is not, which is why the two are separate fields.
They are also the days most likely to be moved for a reason other than a rest day — a mourning period, a coincidence with a religious festival, or a decision to bridge to a weekend.
A national day is an occasion. Whether it is a public holiday is a separate fact, and in several states it is not.
See also public holiday · observance · observed but open
We hold this as category: commemorative, with operational_status carrying what closes. See what we hold.