How we cite our quotes:
Close bosom-friend of the maturing sun; (line 2)
The word "maturing" reminds us that the sun is figuratively growing "older," its rays are getting weaker and the days become shorter.
while thy hook (lines 17-18)
Spares the next swath and all its twined flowers:
The hook is used for reaping but is also associated with death, i.e., the "grim reaper." But Keats softens the blow of this image by "sparing" the next patch of flowers, at least for now.
Then in a wailful choir the small gnats mourn
Among the river sallows, borne aloft
Or sinking as the light wind lives or dies; (lines 27-29)
The language of mortality is extremely subtle in this poem. The gnats are like a funeral choir, singing a requiem for the "dying" day. Also, the wind itself "lives or dies." The speaker has death on his mind in the final stanza.