C'est le lundi et le vendredi.
Let's break it down:
* C'est - It is
* le lundi - Monday
* et - and
* le vendredi - Friday
You can also say:
* Il a lieu le lundi et le vendredi. (It takes place on Monday and Friday)
* Il est prévu le lundi et le vendredi. (It is planned for Monday and Friday)
The specific phrase depends on the context of what you are referring to.