one day (a 24-hour period or a single day); used as a time duration (reading: いちにち).
旅行で一日京都に滞在した。
I stayed in Kyoto for one day on the trip.
会議は一日で終わる予定です。
The meeting is scheduled to finish in one day.
この薬は一日一回服用してください。
Please take this medicine once a day.
彼は一日で仕事を片づけた。
He finished the job in one day.
all day long; the whole day (used adverbially: e.g., 一日中).
子どもは一日外で遊んでいた。
The child played outside all day.
今日は一日雨が降っていた。
It rained all day today.
試験勉強で一日中図書館にこもった。
I spent the whole day holed up in the library studying for exams.
the first day of the month (reading: ついたち); same kanji but different pronunciation depending on context.
来月の一日は土曜日です。
The first day of next month falls on Saturday.
一日は特別な行事があり、会社は休みになります。
There is a special event on the first day of the month, and the company will be closed.