autumn day; fall day
Refers to a day in autumn, often with poetic or seasonal connotations.
An autumn day is like a well-bucket dropping; the sun sets quickly.
autumn sun; fall sun
Refers to the sun in autumn, often associated with its gentle warmth or early setting.
The autumn sun is shining through the window.
Phrase composed of 秋 (autumn) + の (possessive particle) + 日 (day/sun). The reading ひ for 日 is standard in this context.