Literally, it seems that "Stopping By Woods on a Snowy Evening" takes place on the evening of the winter solstice. The winter solstice in the Northern Hemisphere takes place around the 21st of December each year. The winter solstice is the 24 hour period when the Northern Hemisphere has the shortest amount of daylight for the year and the longest amount of darkness. We can guess this because the poem takes place in the winter, when the woods are snowy and the lake is frozen. The solstice would literally be the "darkest evening of the year," because it would be longest night of the year.

Figuratively, Frost also has a tendency to pair what he sees in nature with spiritual or emotional occurrences. If we were thinking of a metaphorical meaning here, the "darkest evening" for a person might be the time when they felt the worst. Perhaps this person is struggling with depression, sadness, anger, hatred, etc. The rider in the poem chooses not to stop and stay beside the snowy wood because he has "miles to go before I sleep" (line 15). You could interpret this sleep as death, so perhaps the rider has decided to not give into depression/suicidal thoughts etc.

Depending on how you read this poem, you might see this as a poem about someone who is tempted to stay and watch the beauty of nature, but must return to his human responsibilities, or about someone who is on the brink of suicide or depression and who chooses to continue his struggle for life.

The "darkest evening of the year" occurs on December 21 in the northern hemisphere. This is the day when there is the least daylight due to the tilt of the Earth's axis. Therefore, December 21 also has the most hours of darkness; after this point, the daylight hours gradually begin lengthening again. This day is known as the winter solstice. Based on the speaker's remark and the contextual clues—it's snowing, and the lake is frozen—it is clear the the poem takes place on or near the water solstice. At a basic level, the detail of "the darkest evening" helps to establish the poem's setting.

Many people interpret this poem as a serene account of a man's journey through a beautiful forest. From this perspective, the darkness of the night symbolizes the unlikely and unexpected nature of the speaker's experience. Indeed, the poem creates an implicit contrast between the speaker's decision to stop and appreciate the stunning wintry woods and the surrounding circumstances that would make such a decision unlikely: the speaker is traveling towards some destination, and the coldness and darkness of the night would make such a stop seem impractical and even ill-advised. Indeed, the horse's apparent reaction—giving "his harness bells a shake / To ask if there is some mistake"—underscores this contrast. At the same time, however, this contrast may also produce some of the woods' aesthetic effect. The speaker seems to appreciate the beauty of the woods precisely because they stand outside the sphere of routine quotidian behavior, in opposition to the "promises" the speaker must keep. Indeed, the unlikeliness of stopping by these woods on the darkest night of year renders them all the more beautiful.

Another way to read the symbolism of the night's darkness is to consider it in light of the relatively common interpretation that the appeal of the woods is connected to the appeal of death, specifically suicide. To read the speaker's journey as a metaphor for the journey of life—a common literary trope—then the speaker's desire to stop midway on his journey, potentially at the expense of his final destination, can be read as a metaphor for the lure of suicide. From this perspective, the fact that it is the darkness evening of the year symbolizes the nearness of death, itself a kind of darkness.

Frost uses these descriptive terms to create a sense of atmosphere. The word "darkest" is applied to the evening: this term is a superlative, meaning that something that is "darkest" is darker than anything else. Because this is not only a dark evening, but the darkest of the year, then, Frost is emphasizing the strangeness of being out on such an evening, stopping in the woods.

In the final stanza of the poem, Frost reiterates the fact that the woods are "dark," but on this occasion it is clear that the darkness is not something to be feared—on the contrary, the fact that the woods are both dark and deep seems "lovely" to him. This creates a sense that the dark depths of the woods are something comforting, enfolding, like the blanket of snow which covers them. The speaker creates a haunting sense of place with these terms: we picture him and his horse enjoying the dark quietude of the woods, and not wanting to leave, and yet being drawn ever onwards by the "promises" they have to keep.

Before answering this question, it helps to check out the final stanza in Robert Frost's "Stopping by Woods on a Snowy Evening":

The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep. (13-16)

Many scholars, critics, and readers have proposed that this seemingly simple poem is actually a depiction of an individual contemplating death and/or suicide. One of the factors that points to such an interpretation is this final stanza, especially the first line: "The woods are lovely, dark and deep." The darkness Frost references, especially when combined with the reference to sleep, seems to symbolize a kind of death. Moreover, the idea that the speaker of the poem eschews such darkness based on the fact he has "promises to keep" suggests that he has some control in choosing whether or not to give into the referenced darkness. This fact could be seen as a suggestion of suicide and, once that idea comes into one's head, it becomes difficult to read the poem and its suggestion of darkness as anything else but a contemplation of death.

That's not to say that this is the only way to read the poem. The meaning of darkness remains fairly ambiguous and, while it certainly could refer to death, it could just as easily refer to something else entirely. Some readers, for instance, point out that the speaker could be talking about rest in general, and so one might not need to wrestle with death at all. In short, there are many readings of this poem, and many interpretations of its "darkness." That said, the idea that darkness suggests death or suicide has proven to be one of the poem's most long-lasting interpretations.