It might be better to use both halves in answeering your question. The first half, "Dark Night" refers to period of personal upheaval or crisis. Both halves: Dark Night, Early Dawn (DNED) is a phrase whose usage probably best elucidates the fuller context behind what a Dark Night exactly is. Transpersonal Studies usage can be summed up in the following;
"Dark Night...", as in Dark Night, Early Dawn typically can be used to refer to a period of great spiritual testing and, when coupled with "...Early Dawn", transformation. For more you might gloss over the book Summary of central metaphor in Dark Night, Early Dawn
Frequently a Dark Night is a period in which the mettle of a given subject is tested to such an extent as to not only unwind the topical or superficial egoic structures of the person, but when harnessed the crisis energy can additionally be utilized to unwind and deconstruct the foundational and subterranean structures.
Also, the actual crisis or context may not objectively warrant such a description, but the diligence and energy channelled into the situation and egoic structures involved would be sufficient to be a Dark Night experience to the subject.
When used in the superlative, there is an understanding of the Darkest Night laying bare the subject's entire self as a collection of dis-integrated parts. There is, in the sense of the Early Dawn, an opportunity to collect these fragmented elements of the person and to integrate them into a more cohesive personhood.