The time setting for this poem is obviously very important, as the poem's first line attempts to create a context and environment for the story. "Once upon a midnight dreary" tells the reader is it very late at night, and that the weather is dreary. The speaker is pondering "weak and weary" (reading and studying amid a number of books, "many a quaint and curious volume of forgotten lore" which implies books of history or folklore perhaps). The speaker is tired, perhaps ill, and feeling weak, which gives an impression of vulnerability. Coupled with the extremely late hour, the vulnerable physical and mental state of the protagonist gives the poem a particularly uneasy imagery. The raven's appearance in the middle of the night seems even more likely to be a harbinger of doom, than if it had arrived in the daytime.
Midnight is traditionally known as "the witching hour" or a time when hauntings and supernatural happenings are more likely to occur. Since the narrator is reading books referred to as "quaint," "curious," and "forgotten lore," this suggests he is already open to occult and supernatural topics. His emotional state is also fragile because he is mourning his dead lover Lenore. The season and weather also add to the eerie atmosphere: "Ah, distinctly I remember/ it was in the bleak December/ and each separate dying ember/ wrought its ghost upon the floor." The fire is dying, making the room grow slowly darker, and the winter weather suggests howling winds outside, also creating a frightening atmosphere. These rich details of time, season, weather and situation all combine to give the poem an unsettling mood associated with the horror genre.