|The Annunciation by Fra Angelico|
According to a book I've been reading recently, the ancient church had already set March 25 as the date of Jesus' conception, because the spring equinox was also believed to have been the date of the creation of the world (when light and darkness are equal). But it was also thought to be the date of Jesus' crucifixion---and so creation, incarnation, and atonement coincided on the older dating of the spring equinox.(1)
1. Greg Pennoyer and Gregory Wolfe, eds., God For Us: Rediscovering the Meaning of Lent and Easter (Brewster, MA: Paraclete Press, 2014), 143.