What is the difference between ある日 and ある日のこと?

Question

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? and What is “koto” used for?