adverb
always; constantly; in all circumstances
Describes unwavering, often quiet support or presence; literally 'in shadow and in sunlight', implying through good times and bad. Often used for someone who helps or watches over another without fail.
彼女は陰に日に彼を支えてきた。
She has supported him through thick and thin.
両親は陰に日に私の成功を願っている。
My parents are always wishing for my success, no matter what.
Compound of 陰 (shade, hidden side) + に + 日 (sun, day) + に, literally 'in the shade and in the sun', implying constancy across all conditions. The exact historical origin is uncertain, but the phrase is used idiomatically in modern Japanese.