I've not seen it on every voyage but they do a military appreciation gathering on every voyage. It's usually held in the main theater and they honor the various branches of the military.
Perhaps someone whose been can elaborate but honestly I've never been, its a little too emotional for me.
I've attended the ship gatherings on 9/11 and veterans day and even that leaves me with a lump in my throat and sweat in my eye.
This seems to just be carrying that theme throughout the day with extra trivia events with patriotic themes and the large flag being displayed. I think it's just something they are doing for the journey cruise.
I asked uncle Google to make sure it wasn't actually national military appreciation day and not just a ship thing. It is not but it is national navy day.
It's also national American beer day. And national breadstick day. I don't drink beer, are breadsticks available anywhere?