日に日に

From Old Japanese.

Literally “on the day on the day”, with particle に (ni) indicating time or location.

• (Tokyo) ひ​にひに [híꜜ nì hì nì] (Atamadaka – [1])[1]
• (Tokyo) ひにひに [hì ní hí ní] (Heiban – [0])[1]
• IPA^(key): [çi ɲ̟i çi ɲ̟i]

日(ひ)に日(ひ)に • (hi ni hi ni) 

1. day by day, gradually

   Synonyms: 日増しに (himashi ni), 日を追って (hi o otte)

   工(こう)事(じ)は日(ひ)に日(ひ)に進(しん)捗(ちょく)している。 ― kōji wa hi ni hi ni shinchoku shiteiru. ― Construction is progressing day by day

2. every day

   Synonym: 毎日 (mainichi)

   • 1912, Tōson Shimazaki, Chikumagawa's sketch:

     わたしは同(どう)君(くん)に頼(たの)んで画(が)家(か)の用(もち)いるような三(さん)脚(きゃく)を手(て)に入(い)れ、時(とき)にはそれを野(や)外(がい)へ持(も)ち出(だ)して、日(ひ)に日(ひ)に新(あたら)しい自(し)然(ぜん)から学(まな)ぶ心(こころ)を養(やしな)おうとしたこともある。

     Watashi wa dōkun ni tanonde gaka no mochiiru yō na sankyaku o te ni ire, toki ni wa sore o yagai e mochidashite, hi ni hi ni atarashii shizen kara manabu kokoro o yashinaō to shita koto mo aru.

     I also used to ask him to get a tripod which a painter would use and sometimes take it out in the open to try to cultivate a new mind to learn from the nature every day.

Literally “on the day on the day” or “day by day”, with particle に (ni) indicating time or location.

日に日に (pi₁ ni pi₁ ni)

1. every day
   • c. 759, Man’yōshū, book 11, poem 2397:

     蹔不見戀吾妹日日來事繁

     simasiku mo mi₁nuba ko₂posiki₁ wagi₁moko₁ wo pi₁ ni pi₁ ni kureba ko₂to₂ no₂ sige₂ke₁ku

     When I didn’t see my love for a while, I then missed her; when I come to her every day, then rumors are frequent.

• Japanese: 日に日に (hi ni hi ni)