Holiday vs Holidays

Greetings.

I am confused by the words 'holiday' and 'holidays'. Regardless of pertaining the singular and the plural cases, I would like to know is it right or is it wrong to use the words in these ways :

Holidays :
[...]since it was during the school holidays...

Holiday :
[...]since it was during the school holiday...

I thought the first sentence was correct, until a friend of mine approached me and corrected the first sentence into the second sentence. it would be nice if someone could explain it to me.

Thank you