What is the mood in "The Raven"?

The mood in "The Raven" is dark and melancholic. The first line sets this tone:

Once upon a midnight dreary, while I pondered, weak and weary . . .

A further look reveals more words used by Poe to create an atmosphere of gloom and despondency: "grim," "ghastly," "bleak," and "haunted." We're left in no doubt from the outset that the narrator's feeling pretty down. It all begins in December—a dark, cold, unpleasant month; when "each separate dying ember wrought its ghost upon the floor."
After the raven first appears, the narrator sees the bird as a source of humor. Straight away, his flagging spirits are revived:

Then this ebony bird beguiling my sad fancy into smiling,By the grave and stern decorum of the countenance it wore.

But not for long. There's only so much fun that can be derived from looking at the bird's stern, serious face. Far from being a figure of fun, the raven's continued refrain of "Nevermore" serves to remind the narrator of his lost love and how he's never going to win her back. The mood of the poem continues to become ever more despondent as the narrator is whipped up into a state of melancholic hysteria by the raven's constant refrain.