noun
autumn day; fall day
Refers to a single day in autumn. Often used in poetic or literary contexts.
秋日の穏やかな午後、公園を散歩した。
On a calm autumn day, I took a walk in the park.
noun
Refers to the season of autumn itself, not a specific day. More common in literary or poetic usage than everyday speech.
秋日は紅葉が美しい季節です。
Autumn is a season when the autumn leaves are beautiful.