We don't celebrate because we are immune to tragedy for a day, because we are not. Tragedy finds us on December 25th just like it does every other day. The birth of the baby in the mess and stink of the stable reminds us that God comes into the mess of our lives, even when everything else about them stinks. God doesn't wait until we get our act together, but meets us where we are, as we are, even if that means God gets (his) hands dirty and (his) heart broken. Babies don't wait, and neither does God. This Christmas God will be present on the battlefield, in hospitals and nursing homes, and prowling the streets in patrol cars.
Just like every other day.
That, really, is what Christmas is about: the birth of God into the mess - our mess - where God loves us, helps us, and when need be, suffers with us.
You just know God must love us an awful lot, to go through all that with us.