I think it is when your soul is stripped of the very foundations of belief. (ex: kindness is a weakness and the bad guys rule the world).
The the despair and anguish so deep it is fathomless. So bleak the soul is stripped of anger, hate, hope, joy, love, etc. and no longer even tries to protest the unprovoked misery. The only thing left to feel is pain, it fills the soul completely. The soul is lost in this bleak and profoundly dark abyss until it is rescued by the faith of the heart (it never leaves). The heart remembers love and beauty and that God is, was, will always be.
That is how St John of the Cross could write with joy and wonder at the embrace of love, the beauty he wrote of he found in his faith and love of God. Love so complete and cherished all the more because of the time when the soul was so filled with pain, every other feeling fell away.
The "dark night of the soul" is imprinted forever after, not the pain as much as the moving experience of God's embrace of love. I think the the darker the night, the more wonderful and cherished the feeling of God's love and closeness. God wipes away the pain and the soul shines, the world is filled with beauty even if that world is a place of suffering.
I believe this kind of elevation of the soul is a one time unique event. The circumstances that bring about such elevation of the soul could last for years, maybe even a lifetime.
Theo,
No one can say with absolute certainty, whether there are more than one dark nights or not. One thing I have learned is to take someones pronouncements of absolute certainties with a grain of salt. When I was taking the Rosicrucian studies, I remember the monographs declaring, that we should be walking question marks in our search for answers. It is not a sin to question the veracity of mystical studies. We should not take everything on faith. We should also seek within for the answers. The studies can prepare you to learn about things through direct experience, or they can just be an intelectual pursuit. In my postings on this web site, I try to keep to things which I have first hand knowledge of. I feel that many students have had plenty of theory, but not enough experience. If the students hear from one who has had many of the experiences they read about,and hear about my experiences in reaction to mystical teachings, it may give them a better idea of things. Not that they should try to mimic mine, or anyone elses path, for we are all experiencing things from our own perspectives which will not be the same from one person to another. When I was younger, I used to always wonder where I was in the scheme of things, and compare myself with those who were more advanced than me. This is a fruitless exercise. Follow your own path and dont worry about someone elses. Sometimes I visualize that the path to enlightenment is a ladder of a hundred steps reaching into the heavens. During my own Journey, sometimes I have thought I was halfway up the ladder, then at other times (after a spectactular failure), I was on the bottom rung barely hanging on.
Gary