Our speaker thinks he saw his deceased wife, but he isn't exactly sure. Also, he's blind. So we're not exactly on firm ground in "Methought I Saw my Late Espoused Saint" when it comes to figuring out if this woman is a figment of his imagination, a dream, a ghost, or who knows what? At the end of the poem, the speaker tells us that he "wak't," giving us our first indication that what we thought was a vision or a spirit visiting may actually have been a dream. Maybe the "Late Espoused Saint" is no saint after all, but just a passing dream.
Using a series of similes to describe his vision tells us that this woman not actually the speaker's deceased wife, but something like her.
"Day brought back my night" draws a contrast between the speaker's dream reality and his current one, and he's not a fan of waking life.