The story
Navarātri is nine nights of a single fight: Mahiṣāsura, a demon who could not be killed by any god or man, against a goddess assembled out of the anger of all of them. The eighth night is when it turns.
Bengal reads it through Durgā coming home to her parents with her children for five days, and the eighth day is the height of that visit. The Sandhi Pūjā falls at the joint of the eighth and ninth tithis, forty-eight minutes long, and the whole pandal waits for it.
In the north the day is Kanyā Pūjā: nine young girls are seated, their feet washed, and fed as the goddess herself.