The ancient mystery of the "sun at midnight," symbolizes the spiritual light that lies hidden within the dark. The Celts knew that light emerges out of darkness and so their days began at dusk, as if the sun was seeded in the black earth of night.
The dark night of the soul refers to an extended period of acute purification that a spiritual practitioner undergoes immediately before making the final transition to deep spiritual awakening. It emphasizes purification and the act of letting go of what no longer serves after many lesser trials have been navigated.