Celebrated: In the United States since 1866
Because Memorial Day isn't about celebrating military service, or even honoring those who serve. It's about remembering the dead, whether or not we believed in the cause they fought for.
According to legend, after the battle of Fredericksburg, General Robert E. Lee turned to Lt. General James Longstreet and said, "It is well that war is so terrible—we would grow too fond of it." You'll find several different versions of this quotation out there. Military historians love it, but it wasn't recorded at the time, and was first cited in a biography of Lee published five years after the war. Maybe he said it, maybe he didn't. It gets quoted because it's true.
My thoughts about these things are not well organized, after a week that included the President's remarks on drone strikes and a viewing of the ultra-violent latest Star Trek movie. Humans have always turned war into entertainment. I could argue that The Iliad was the ancient Greeks' version of a summer blockbuster. But no matter how you glitz it up or adorn it with high principles and sacred honor, the one permanent truth about war is that it kills people. People die, and they leave parents and spouses and children to mourn them. We cannot allow ourselves to forget this. We cannot fool ourselves that anything we do — drone strikes, for God's sake! — absolves us of the personal moral responsibility we all bear for joining in or supporting any enterprise that is going to get people killed.
I'm not saying war is never justified. I grew up in a military household, and I do believe some things are worth dying for. But Memorial Day is a day for us to count our losses, to face them with all the wild grief they deserve, so that we can honestly answer the question of what might be worth more of those losses in the future.