The word "a" can mean different things. It can mean something specific or something general. For example, the phrase "a movie" can be used specifically:

I saw a movie last night.

Or it can mean something general:

It's fun to go out to see a movie every once in a while.

But when you use the phrase "a certain ___", it's clear that you mean a specific thing.

I made the mistake of telling a certain friend about my problems with Jeff, and next thing you know the whole school was talking about it.

Let's just choose a certain day to meet for a few hours each week.