This is the 14696th most frequent German word.
nationaler
"Nationaler" means "national" in English, describing something related to a nation.
The word "nationale" is used here as an adjective describing the holiday, specifying that it pertains to the nation.
Der nationale Feiertag wird jedes Jahr gefeiert.
The national holiday is celebrated every year.
The word "nationalen" is used here in the dative case to describe the competition.
Er hat an einem nationalen Wettbewerb teilgenommen.
He participated in a national competition.
Here, "nationale" is describing the monument, indicating its significance on a national level.