Two probable interpretations:-
1) It was the Winter Solstice..
2) Darkness signifies MYSTERY. There was silence in the woods, and SILENCE can intensify DARKNESS. You might have experienced it , at times of any festival, music and other noises goes on till late night. At that situation you would hardly feel that it's midnight. And if you happen to be all alone in your locality in one evening, with complete SILENECE, no sound of people, THAT EVENING WOULD SURELY APPEAR TO BE LIKE MIDNIGHT FOR YOU..
So, basically it was the silence that was intensifying the darkneas of that area. And that is why he called it the "darkest evening".