One day he just upped and left.

有一天他突然离开了。

《柯林斯英汉双解大词典》

One day, I'll be rich and famous.

总有一天我会名利双收。

《牛津词典》

Maybe one day you'll find true love.

也许有一天你会找到真爱。

《柯林斯英汉双解大词典》

As he tells it,one day he looked in the window of a school and noticed how dark it was.

VOA: special.2009.12.21

for why you became afraid of dogs is that one day a dog came up and he was a neutral stimulus.

你之所以害怕狗,是因为，某天有只狗向你跑来,狗是一个中性刺激。

耶鲁公开课 - 心理学导论课程节选

and then one day I'll be like, "Oh, I want to dress like a clown today, so I will."

而可能有时候，我想，“喔，今天我想打扮成一个小丑，然后我就这么做了。”

时尚让人兴奋 - SpeakingMax英语口语达人

Another artist joked morbidly that one day Picasso would be found hanging behind "his giant canvas".

BBC: Too famous to see?

Then one day the haze lifted and I saw the Pamirs for the first time.

BBC: Following the Silk Road to the end of China

This may be one day a week or one day a fortnight they may exclude.

BBC: Couple drinking