How does Keats manage to embody such a complex theme in a simple poem?

One of Keats's greatest poems, "To Autumn," is at first glance a seemingly simple description of a season. Keats describes classic autumn scenes in the first and third stanzas, and in the second he personifies autumn and gives the season life through clever human characteristics. To say that the poem is only a series of descriptions would be missing the point, though. Indeed, the poem is also about the inherent (and, in some ways, surprising) beauty of endings and conclusions.

This idea is most forcefully communicated in the final stanza. First, Keats rejects the "songs of spring" (23), which is itself a symbolic suggestion that beginnings (represented here by the birth of a new year) are not the only valuable stages of a cycle. Keats follows this idea up with a description of a beautiful, but melancholy, autumn landscape:

Then in a wailful choir the small gnats mourn 

   Among the river sallows, borne aloft 

      Or sinking as the light wind lives or dies; 

And full-grown lambs loud bleat from hilly bourn; 

   Hedge-crickets sing; and now with treble soft 

   The red-breast whistles from a garden-croft; 

      And gathering swallows twitter in the skies. (27-33)

This bittersweet description is beautiful, but it also reminds us that the autumn landscape is becoming dormant, and preparing for winter and the end of a year. Often, this period is thought of as wholly melancholy, but Keats suggests that there is value to be had even in the process of ending a year. By extension, we can see Keats arguing through his subtle descriptions that conclusions are not only sad, but also moments of extreme beauty and atonement. Thus, in a very simple poem, Keats muses on the forgotten value of cyclical endings, a topic that becomes even more poignant when we take into account Keats's own untimely death.