One of my desk drawers is filled with old calendars, which I’ve been saving as a prop for a faulty memory. I suspect it’s a fruitless exercise. Appointments and to-do lists, however necessary, don’t add up to a life, and the dates that really do matter return like faithful comets. Among the fairest of all these periodic messengers is March 25, the ancient and universal feast of the annunciation, or Lady Day, as it used to be called. It commemorates the first instant of the incarnation, when the angelic greeting met the maiden’s consent. Everything changed in that instant, as the fifth-century pope Leo the Great observed in a letter to the bishop of Constantinople.