An example of this is that I have seen both 来週テスト and 来週のテスト. Both of them mean "next week's test".
My issue is that I have no clue when to use which or what makes them different from each other. In what situations can you leave out the の?
Edit: Just to clarify, the example I've mentioned above is one of many instances I've heard, as in when listening to shows to better grasp spoken Japanese (opposed to textbook). I'm trying to get a general guideline as to situations when one leaves it in, and when one takes it out.