For example:

> ある日のこと、たろうは、また海へ出て。

As I understand ある日 means "one day". What does the のこと do to it?

  
Possibly related? [What is the こと in sentences such as
あなたのことが好きだ?](https://japanese.stackexchange.com/questions/2102/what-is-
the-%E3%81%93%E3%81%A8-in-sentences-such-
as-%E3%81%82%E3%81%AA%E3%81%9F%E3%81%AE%E3%81%93%E3%81%A8%E3%81%8C%E5%A5%BD%E3%81%8D%E3%81%A0)
and [What is “koto” used
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2019-08-10T12:52:59.777
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2019-08-10T12:52:59.777
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question
word-choice nuances
What is the difference between ある日 and ある日のこと?