We're inside the speaker's head, where he's having a really beautiful, heaven-sent vision of his wife. Kind of like in a dream, his mind wanders all over the place while he's thinking about it.
First, we're taken to the edge of the ancient Greek underworld, where Hercules is snatching Alcestis right out from under the nose of Hades. Then we're whirled ahead in time to the desert encampment of the Israelites, where a woman who has recently given birth is entering a ritual bath to purify herself. Then we're in Heaven, where a woman so beautiful that her radiant soul shines all over the surface of her body is leaning down to embrace us. We wish we could stay here forever.
The speaker wakes, though, bringing us to the bedside of a blind, grieving widower during a sunrise he can't see. Although it's light out, the depth of this man's grief makes his room seem dark and oppressive. Being in his head, and experiencing the loss of his wife all over again with him, helps us to understand what he means when he says that "day brought back my night" (14).