"Darker and darker," concerning a Wendell Berry quote

When Advent arrives each year, we find a flurry of folks quoting and re-quoting this fine sentence, "It gets darker and darker, and then Jesus is born." This is often, but not always, attributed to Wendell Berry. And it is sometimes attributed to something Wendell Berry has written. Yet, to my knowledge, he has not actually used it in any of his works.

The source is, in fact, Wendell Berry via Anne Lamott, who used it in Plan B: Further Thoughts on Faith (2006). Here is the relevant page: