The mood of the poem is quiet and contemplative. The narrator has stopping in the dark evening to admire the snowy woods, and he is far from the village where the owner lives. The village would have lights, horses and pedestrians, movement and sound and life. The woods, by contrast, are dark and quiet, with no sound but that from the narrator's horse:
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
(Frost, "Stopping by Woods on a Snowy Evening," eNotes eText)
Without these distractions, the narrator can focus on the beauty of the woods and the meaning of his own life, spent bustling around "keeping promises" and "traveling miles." He would rather go into the woods and exist in the quiet and dark, but he knows that soon he must return to his busy life. This moment of contemplation is a small joy among the daily grind of work, personal interaction, and obligation.
In the first stanza of the poem, the mood is one of wonder and admiration because the speaker has stopped in the woods to watch the snow fall. This is clearly an enjoyable and interesting activity for the speaker since there is no other reason for stopping in the middle of the woods, as expressed in the second stanza:
My horse must think it queer
To stop without a farmhouse near.
In the third stanza, the mood becomes calm and serene. Alongside the sound of the horse's bells, for example, the only sound that can be heard is that of the "easy wind." This creates a sense of tranquillity which is further reinforced by the closing two lines of the poem in which the speaker comments that there are "miles" before he can sleep. By repeating these lines, the speaker soothes and calms the reader, just as the snowfall seems to soothe and calm his self.