mild late autumn day; Indian summer day
Refers to a warm, calm day in late autumn (around November), similar to an Indian summer day. Often used in the phrase 小春日和 (こはるびより).
See also: 小春
Today is a mild late autumn day, and it feels very pleasant.
The mild late autumn days continue, and the garden flowers are still blooming.
小春日和 is the full phrase meaning 'mild late autumn weather', while 小春日 refers specifically to the day itself.
Compound of 小春 (こはる, 'little spring', referring to a warm period in late autumn) and 日 (ひ, 'day'), with rendaku voicing of ひ to び.