For those most directly involved, the date of December 26, 2004, marks a brutal interruption - the death or injury of someone, terrible anxiety, bereavement, anger and bewilderment. But for all of us, the date will carry significance, for all of us something erupted into our comfortable consciousness. Like 11 September and, now, 7 July as well, it stands in the landscape or the map, a feature that will never be obliterated. That was when things changed.
Anniversaries are among the things we most take for granted in our personal lives. It's all the more strange, then, that so many are so reluctant to treat Christmas as an anniversary.
God forgive us. But if we do ever come to forget not just the Christmas story but what it made possible, the end of Rome, the arrival of a different humanity, there is enough, sadly, in our idle and self-obsessed hearts to let the ancient world begin to creep back a little bit more.
I don't believe that in fact it could be possible to forget. When modern tyrannies have tried to make people forget, memory has shown itself pretty tenacious, secretly, obstinately, subversively.
A few weeks ago, Gee Walker, mother of the murdered Liverpool teenager, Anthony Walker, told us that, yes, she forgave her son's killers and, yes, her heart was still broken. What made this so intensely moving was the fact that her forgiveness was drawn agonisingly out of her, without making her loss easier.
And last week, the mother of Abigail Witchalls, paralysed by a knife attack in April, described her sadness about Abigail's attacker, who had killed himself.
Why remember what happened at Bethlehem, why resist the efforts to reduce it to a brief fling of sentimental goodwill in the middle of bad weather? Because of people like these.
They have known in their flesh and nerves what the difference is that Jesus makes; it is not comfort or easy answers, it is the sheer fact that - we have to use the word - miraculous love is possible. The vilest offender, as the hymn says, is now deserving of attention and compassion; no life can be allowed to fall out of the circle of love.