Gradually; day by day — indicating change or progress little by little over time.
病状は一日一日良くなってきている。
Her condition is getting better day by day.
練習の成果は一日一日積み重なっていく。
The results of practice accumulate day by day.
彼の日本語は一日一日上達しているように見える。
His Japanese seems to improve little by little each day.
Each day; every day — can be used as a noun meaning daily life or the passing of days.
一日一日を大切に生きようと決めた。
I decided to live each day with care.
旅行中は一日一日が新しい発見の連続だった。
During the trip, each day was full of new discoveries.
日記には一日一日の出来事を細かく書いた。
I wrote down the events of each day in my diary in detail.