1An abbreviation (often on schedules or signs) meaning Sunday (used as a calendar/label symbol).
店の定休日は入口に『日』と書かれている。
The shop's regular holiday is written as '日' at the entrance.
掲示板にはシフトの休日が曜日ごとに『月・火・水・木・金・土・日』と表示されている。
On the notice board, days off are shown per weekday as 'Mon, Tue, Wed, Thu, Fri, Sat, Sun' ('日' for Sunday).
学校行事の案内に『日』とだけ書かれている日は日曜日のことだ。
If an event notice shows just '日', it refers to Sunday.
2A suffix attached to numerals to indicate the nth day of a month (e.g., 十五日 'the 15th', 二十一日 'the 21st'). Used for most date numbers except those with special か readings (1st, 2nd–10th, 14th, 20th, 24th).
私の誕生日は六月十五日(じゅうごにち)です。
My birthday is June 15th.
会議は来月の二十五日(にじゅうごにち)に開かれる予定です。
The meeting is scheduled to be held on the 25th of next month.
毎月十一日(じゅういちにち)が給料日だ。
Payday is the 11th of every month.
3A counter for days. 一日 (いちにち) means 'a whole day / 24 hours'; for 11 days and above the にち reading applies (with exceptions like 14, 20, 24, which use か).
この薬は一日(いちにち)三回飲んでください。
Please take this medicine three times a day.
出張は十二日間(じゅうににちかん)の予定です。
The business trip is scheduled to last twelve days.
彼は一日(いちにち)に何時間もスマホを見ている。
He spends many hours a day looking at his phone.
4An abbreviation used to represent 'Japan' in compounds (e.g., 日系, 日中), functioning as a prefix for the country name.
日系企業で働くことに興味がある。
I'm interested in working for a Japanese company.
日中会談が来週行われる予定だ。
Japan-China talks are scheduled to take place next week.
日米の経済関係は深い。
Japan–U.S. economic relations are deep.