Macedon might celebrate something in relation to their pegusi or dragons, if you assume mini pegusi are born at a certain time of year. And maybe they make a point of celebrating most holidays with some kind of war game or tournament because Archanea would do it with poetry, and Macedon thinks they're dumb.

(Gotta say, while I don't like Hetalia much, it'd be hilarious (or depressing?) to do a Hetalia-style rendering of the different nations in Archanea. Someone has probably already tried... for another FE game.)

When it comes to holidays, I tend to go very general (equinox, harvest, etc.) because it's safe. Harvest festivals seem especially logical. Celebrating the coming of spring seems logical. It's not very exciting or creative, though.

Micaiah would get her own saint day. Ike would get five.