天高く馬肥ゆる秋 Ten takaku, uma koyuru aki. The sky is high, and horses grow stout in autumn.
Traditional Japanese proverb
Reference works preserve both the familiar full form and the shorter form 天高く馬肥ゆ. The saying has a historical Chinese frontier context; in modern Japanese, it evokes autumn.
Why this date: The first weekend after 寒露 is a natural point to notice the high, clear autumn sky.
Language note: 天 (てん / ten) is the sky; 肥ゆる (こゆる / koyuru) is a literary form meaning to grow fat. In present Japanese the saying evokes autumn's clear sky and comfortable season.
Some sayings turn the whole season into one vivid scene.
DJW practice example: 澄んだ空を見上げて、秋を感じる。 Sunda sora o miagete, aki o kanjiru. Look up at the clear sky and feel autumn.
Sources: Kanjipedia — 天高く馬肥ゆ: https://www.kanjipedia.jp/kotoba/0005042600 Kotowaza Dictionary — 天高く馬肥ゆる秋: https://kotowaza-dictionary.jp/k0461/ Toho University — historical and seasonal context: https://www.toho-u.ac.jp/sci/bio/column/031384.html