My favorite poem related to fall in the Hyakunin Isshu is this one:

Japanese	Romanization	Translation
さびしさに	Sabishisa ni	When, from loneliness
宿を立ちいでて	Yado wo tachi idete	I stand up and leave my hut
ながむれば	Nagamureba	and look distractedly about:
いづずこも同じ	Izuko mo onaji	everywhere it is the same
秋の夕ゆうぐれ	Aki no yugure	evening in Autumn.

The author of this poem is a monk named Ryōzen Hōshi (良暹法師, dates unknown) or “Dharma Master Ryōzen”, who supposedly composed it while doing austerities in a remote hut outside the capitol. Unfortunately, we have little about Ryōzen Hōshi’s personal history, even in my new book. He had some infamy over a poem he composed during a poetry contest, by inadvertently copying one in the Kokin Wakashu, causing him to be a laughing stock. However, other scant records show he was still respected by the nobility overall.

The notion of “Autumn sunset” appears a lot in Japanese poetry, but apparently its meaning differs depending on the time and place. Ryōzen 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 Ryōzen’s imagery the best, and it’s the one I imagine whenever I read this poem.