My favorite poem related to fall in the Hyakunin Isshu is this one:
寂しさに Sabishisa ni
宿を立出て Yado wo tachi idete
いづこもおなじ Izuko mo onaji
秋の夕暮 Aki no yugure
Which Professor Mostow translates as:
When, from loneliness
I stand up and leave my hut
and look distractedly about:
everywhere it is the same
evening in Autumn.
The author of this poem is a monk named Ryosen Hōshi (良暹法師, “Dharma Master Ryosen) who supposedly composed it while doing austerities in a remote hut outside the capitol.
The notion of “Autumn Sunset” appears a lot in Japanese poetry, but apparently its meaning differs depending on the time and place. Ryosen Hoshi gives a more melancholy, almost Buddhist, tone implying that the world around him is declining into winter and possibly, metaphorically declining in a general Buddhist sense. However, Sei Shonagon (poem 62) also wrote about Autumn Sunset in her Pillow Book, but used it to describe crows and wild-geese flying
An Autumn Sunset means a lot of different things to a lot of different people, but it still is significant one way or another. For me, I tend to like Ryosen’s imagery the best, and it’s the one I imagine whenever I read this poem.