Question

what is the meaning of darkest evening of the year in stopping by the woods

Answer 1

 The darkest evening of the year could be a subjective opinion of the narrator. Perhaps he has something negative weighing down his thoughts.
hope it helped :)

Answer 2

ANSWER:

   By the phrase ‘darkest evening of the year’, the poet, Snowy Evening mentions the darkest night of the year that falls on 21st or 22nd of the month of December. That is the longest night of the year.

  This happens because the sun goes farthest South on that day. By this phrase, the poet can also mean that the speaker had had bad time on those nights or that he was depressed.