(いちにち)

いちにち
noun
one day, all day
1. one day (duration)
A period of 24 hours or one full day.
How many hours do you work in one day?
I spent one day in Tokyo.
The trip will end in one day.
This work won't be done in one day. It takes about three days.
I thought it would be done in one day, but it took two days.
2. all day, the whole day
Throughout an entire day from morning to night.
一日(いちにち)(じゅう)(あめ)でした
It rained all day.
一日中 = all day long
一日(いちにち)(じゅう)(ほん)()んでいました
I was reading books all day long.
I stayed at home all day on Sunday.
子供(こども)たち一日(いちにち)(じゅう)(そと)(あそ)んでいました
The children were playing outside all day long.
風邪(かぜ)()いたので一日(いちにち)(じゅう)()いました
I caught a cold, so I was sleeping all day.
Please rest for the whole day.

一日(いちにち) means 'one day' as a duration. Note the different reading: when meaning 'the first day of the month,' it's read as 一日(ついたち). 一日(いちにち)(じゅう) means 'all day long.' Compare with other duration words: 二日(ふつか)(かん) (two days), 三日(みっか)(かん) (three days).