Hello...I just wanted to make sure that I am supposed to take the first (1 out 6) practice CAT tests before attending my first class session on sunday. It should serve as a diagnostic right?

Exactly! Remember that it takes 4 hours - and do the essays. A lot of people skip them b/c they don't care as much about the score on that portion, but stamina is a HUGE issue on the test. You need to be prepared for that. (If you've already taken your first test and didn't do the essays, that's okay - but do them from now on!)