日(ひ)に日(ひ)にday by day, by the day, daily, every day

生徒(せいと)達(たち)の学力(がくりょく)が日(ひ)に日(ひ)に上(あ)がっている。

The students' academic abilities are improving day by day. {day by day, by the day, daily, every day}

患者(かんじゃ)の容態(ようだい)が日(ひ)に日(ひ)に快方(かいほう)に向(む)かっています。

The condition of the patient is improving day by day. {day by day, by the day, daily, every day}

雪(ゆき)も溶(と)け始(はじ)めて日(ひ)に日(ひ)に春(はる)へと向(む)かっているようだ。

The snow is starting to melt and it seems to be heading towards spring day by day. {day by day, by the day, daily, every day}

ダイエットを始(はじ)めてから日(ひ)に日(ひ)に体(からだ)が軽(かる)くなってきました。

Since I started dieting, my body has been getting lighter day by day. {day by day, by the day, daily, every day}

日(ひ)に日(ひ)に病状(びょうじょう)が悪化(あっか)しているあの患者(かんじゃ)はもうすぐで亡(な)くなるだろう。

That patient, whose condition is deteriorating day by day, is likely to pass away soon. {day by day, by the day, daily, every day}