As always, it's difficult to say what the single most important message of a particular work of literature is, and John Keats' "To Autumn" is no different. However, it is possible to say what one of the major themes/messages is. In general, one can argue that the poem's message focuses on describing the melancholy beauty of the season of autumn and connecting this description to the general beauty of endings and conclusions within the cycles of the natural world.
Throughout the poem, Keats lingers on the beauty of the natural world during autumn. However in the last stanza, he more forcefully connects autumn to the beauty of endings (or death) within the natural world. For instance, Keats says, "Then in a wailful choir the small gnats mourn / Among the river sallows, borne aloft / Or sinking as the light wind lives or dies" (27-29), and these lines reference the "death" that autumn ushers in in preparation for winter. However, while melancholy, Keats sees this natural "death" as beautiful in its own right, as it follows a productive harvest that symbolizes a fruitful existence. Keats underscores this melancholy positivity by infusing even this last stanza with exceptionally beautiful natural imagery.