日日夜夜的英文：
day and night; night and day; by day and by night

参考例句：

The days and nights were a blur of medical reports.
日日夜夜都是治疗报告。

For all the nights that were filled with dread, and for the worries I knew were ahead, No Charge.
为充满恐惧和担心的日日夜夜，一分都不收。

These men bear this problem in their hearts and their hearts and their brains, by day and by night
这些人日日夜夜把这个问题放在心中，记在脑里。

day是什么意思：
n. 一天；白昼，白天；工作日；时代，时期

Things go on day by day.
事情逐日进行着。

Day by day the situation is improving.
情况在一天天地好转。

Like this, day after day.
就这样，日复一日。

It was a fateful day.
那是个不祥的日子。

Meantime, from day to day they endure.
同时，一天天他们捱了下去。

night是什么意思：
n. 夜晚；黑夜；黑暗
adj. 夜晚的，夜间的

It was an obscure night in early May.
那是五月初的一个朦胧的夜晚。

The landlord bestowed him for the night.
店主留他住一夜。

They pitched up for the night.
他们扎营过夜。

Good night, old sport.
晚安，老兄。

An owl is the king of the night.
猫头鹰是夜晚之王。