One way to look at this poem is through the theme of the contemplation of life. In the poem, the speaker has paused in the middle of the woods to look at the landscape and admire its beauty. Note that, in the last stanza, the woods are "lovely, dark and deep." Foremost, they are lovely. Had Frost put a comma after "dark," each term would have been given equal weight. However, he singles out "lovely" first and then adds that the woods are "dark and deep" as well. So, in this moment of contemplation the notions of "dark and deep" might not be interpreted in negative ways. The speaker could be remarking that the entire scene (snow, darkness, and depth) as a combined whole is lovely.
On the surface, this is a moment of contemplation of an evocative landscape. But note - it is evening. This is a time after work hours and before actual night. It is a transition period. It is the darkest night of the year. That means it is December 22nd, the winter solstice. This is also a transitional date, moving from autumn to winter. So, in this moment of contemplation, the speaker must be thinking about the past and the future. Such is the nature of being in a transitional phase. Given that it is evening, he is not preoccupied with work, which would occur earlier in the day. And given that it is prior to nightfall and he is not home yet, he is not with his family or going to sleep. He is in between these two phases of the day. It is a transitional part of the day and year.
Consider this as a parallel to his life. The early part of the day (youth) is past and the night (death) is yet to come. He is perhaps in the midlife stage. This is a time for reflection. He wonders about what he should or would have done in the past. He then looks forward to the promises he has to keep for the future. So, this thematic analysis of contemplation shows how Frost is using a moment in the woods to evoke other notions of reflection and contemplation in a transitional yet fleeting stage.