What is the setting of the poem "The Road Not Taken"? That is, where is the poet and what time of the year is it?

The poet is walking in a forest, and the time of year is Autumn.

Since the poem was published in 1916 only shortly after Frost left England where he would often take walks with his close friend, Edward Thomas, the forest is probably near Gloucestershire, where Frost owned a cottage. However, since he had already returned to New England at the time of the publication of his poem, Frost could have just as easily used...

The poet is walking in a forest, and the time of year is Autumn.

Since the poem was published in 1916 only shortly after Frost left England where he would often take walks with his close friend, Edward Thomas, the forest is probably near Gloucestershire, where Frost owned a cottage. However, since he had already returned to New England at the time of the publication of his poem, Frost could have just as easily used a New Hampshire forest as his setting. The time of the poem is probably between 1912 and 1915 while he was a close with the indecisive and troubled Thomas. For, Frost himself declared that the poem was composed with Thomas in mind. With the description of a "yellow wood," the time of the year would be the fall. 

While a description of the indecisiveness of Edward Thomas may be the objective of the poem, there have been many metaphoric interpretations of this popular verse, especially one that involves faltering at an important time of one's life. This interpretation may still return to Thomas who chose to go to war rather than accompany Frost to New Hampshire. So, just as the speaker of the poem hesitates--

And both that morning equally lay In leaves no step had trodden black. Oh, I kept the first for another day!

Thomas faltered in his friendship, the most important of either man's life, and remained in England, enlisting as a soldier.