All compounds with "-tag" use "an", and a few other cases are included by extension:
am Tag
am Montag, am Dienstag -> am Mittwoch
am Mittag -> am Vormittag, am Nachmittag -> am Morgen, am Abend
Nearly everything else uses "in":
in der Nacht
im Januar
in der nächsten Woche
im Jahr 2001
And of course "um" if the time is given.
I'm not sure what you mean by "Nationaltag", maybe "Nationalfeiertag"? Anyway, if it's a compund with "Tag", you use "an". There are several national holidays, so of course you can use the plural if you mean all or several of them.