1One day — a 24-hour period or a single day, used as a unit of time in expressions like 一日に〜回 ("~ times a day") and 一日かけて〜 ("taking a whole day to ~"). The standard reading is いちにち; いちじつ is the same meaning with a more formal, literary tone. (The reading ついたち, "first of the month," is a separate dictionary entry that happens to share the same kanji.)
この薬は一日に三回飲んでください。
Please take this medicine three times a day.
京都には一日しかいられなかった。
I was only able to stay in Kyoto for one day.
この仕事は一日では終わらない。
This job won't be finished in a single day.
一日24時間では足りないくらい忙しい。
I'm so busy that 24 hours in a day isn't enough.
2(Read いちにち) Used adverbially to mean "all day," "throughout the day," or "from morning till night." Often appears in the form 一日中 / 一日じゅう, indicating that a state or action lasted the entire day.
今日は一日中雨が降っていた。
It rained all day today.
子供は一日外で遊んでいる。
The kids are out playing all day.
風邪をひいて、一日寝ていた。
I caught a cold and stayed in bed all day.
試験前は一日中図書館にこもっていた。
Before the exam, I shut myself up in the library all day.
3The first day of the month. In this sense the entry is read mainly いちじつ, appearing in contracts, official documents, and historical writing. Everyday speech uses ついたち (a separate dictionary entry), while certain business documents use いっぴ.
本契約は令和7年4月1日（いちじつ）をもって発効する。
This contract shall take effect on April 1 (ichijitsu), Reiwa 7.
明治5年12月1日（いちじつ）、新暦が施行された。
On December 1 (ichijitsu), Meiji 5, the new calendar took effect.