Where It All Goes Down
We've said it before, and we're going to keep saying it. "Loveliest of Trees" was first published in 1896 a book called A Shropshire Lad. Most of the poems (including "Loveliest of Trees") in the book evoke the relatively rural environs of Shropshire, which is a county in western England along England's border with Wales, as you can see here.
Even in the late nineteenth century (when Housman wrote "Loveliest of Trees"), Shropshire was still a pretty rural place—lots of open country, no cities as big and dirty and metropolitan as London. It was the kind of place where you could definitely take a ride on your horse and look at a bunch of cherry trees, like the speaker of the poem does.
While the poem conjures up this quiet, peaceful world, you could also say that it paints a picture that we'll just call the setting of life. The blooming cherry trees of the first and last stanzas indicate that it is late spring (probably sometime in April).
For his part, our speaker is still in the "spring" of his life—he's only 20 and has a lot of living yet to do. Despite his young age, however, the speaker is also looking towards his "winter": old age and death. For him, it is virtually impossible to even think about spring without also thinking about winter. All those references to the color white, and, of course, to snow, make it very clear that winter is as much on the speaker's mind as spring.
The seasonal imagery of the poem, then, is matched by the speaker's own reflections on youth and old age. In other words, even though this poem is about a cherry tree in Shropshire, it's really about more than that. That cherry tree in Shropshire is actually a metaphor for the "woodlands" of life.