Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
On the surface this refers to the  winter solstice  here, but upon closer examination it may have, well, darker connotations. 
 The “darkest evening of the year” could suggest the speaker’s own psychological state. Perhaps he has something negative weighing on him, taking his mind to darker places than it’s gone before. Some critics have even read this as a reference to suicidal despair–a reading that  Frost publicly resisted: 
 
 Indeed, critics sometimes set his teeth on edge with intimations about personal themes in the poem, as if it expressed a wish quite literally for suicide or marked some especially dark passage in the poet’s life. Louis Mertins quotes him in conversation (and similar remarks may be found in transcripts of a number of Frost’s public readings): 
 “I suppose people think I lie awake nights worrying about what people like [John] Ciardi of the  Saturday Review  write and publish about me [in 19S8]….He makes my ‘Stopping By Woods’ out a death poem. Well, it would be like this if it were. I’d say, ‘This is all very lovely, but I must be getting on to heaven.’ There’d be no absurdity in that. That’s all right, but it’s hardly a death poem. Just as if I should say here tonight, "This is all very well, but I must be getting on to Phoenix, Arizona, to lecture there.” 
 –Mark Richardson,  The Ordeal of Robert Frost: The Poet and His Poetics  (1997) 

Stopping by Woods on a Snowy Evening