I also cannot figure out why [South Korea's Memorial Day focused on commemoration of Korean War dead] falls on June 6th, which, as far as I know, has no significance to Korean War history.
We in the USA have two veterans' holidays, November 11th being WWI Armistice Day, many will know. We also have what we call Memorial Day. Why is our Memorial Day at the end of May? I cannot figure out a reason for that.
Memorial Day in the USA's real function, of course, is marking the start of Summer. So it's not a "veterans" holiday at all, except highly superficially. Look at what really happens on Memorial Day (in the USA), what people actually do. They get their "summer routines" started, with barbeques, good-weather-oriented overnight trips (my aunt and cousins were so engaged this year, as they often are on Memorial Day weekend, travelling to Connecticut), or perhaps doing yard-work, or going bicycling, or picnicking. Swimming pools open on Memorial Day weekend.
None of the things regular people do on the USA's Memorial Day are connected with veterans' commemoration, it seems to me. What they actually do is connected with "commemoration", or celebration, of the seasonal change; the start of consistent good weather; the start of summer.
The USA's Memorial Day, it occurs to me, is a "seasonal-change holiday", just like the old pagan festivals. It appeals to the same instincts as the European pagans did with their seasonal festivals. My own name, and the name of this blog, is derived from such a Northern-European pagan seasonal festival.
Old habits die hard. I don't think this is a bad thing.