Question

From “The Raven” by Edgar Allan Poe Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten lore— While I nodded, nearly napping, suddenly there came a tapping, As of some one gently rapping, rapping at my chamber door. “’Tis some visitor,” I muttered, “tapping at my chamber door— Only this, and nothing more.” In the verse, which line indicates the time when the events of “The Raven” occur?

Answer 1

It happens at midnight, or late at night, "Once upon a midnight dreary"

Answer 2

The line which indicates the time when the events of “The Raven” occur is the following one:

"Once upon a midnight dreary..."

It is an example of an adverbial phrase, which answers the question: "when?". It cannot be simply called an adverb, since it does not consist of a single word, such as yesterday, tomorrow or today. It is rather an adverbial phrase because it consists of a group of words that work as an adverb. In the case at hand, it functions as an adverb of time.