ある before nouns


I was wondering what is the nuance with ある before nouns, like in this
sentence.

> 夏の **ある** 暑い日の午後、彼はその運転手に再会した.

I don't quite understand if the addition of ある before nouns can convey a
certain feeling, or certain nuance/meaning that I'm not aware of. To me, 夏の暑い日
and 夏のある暑い日 can be translate basically in the same thing. Other time a saw
this kind of thing, especially in novel.

So ある, in these cases, what add in particulary?

