In Robert Frost's "Stopping By Woods on a Snowy Evening," line 8 of the poem mentions that the events of the poem take place on the "darkest evening of the year."
There are two possible meanings here: a literal meaning and a figurative meaning.
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.