Questions about hospital scheduling

I'm a first semester nursing student. I've been working as a MA in a doctor's office for a year and a half but am trying to get a job at a local hospital as a PCA. I have two interviews scheduled next week. I have a few questions.
Is hospital scheduling different each week? Or do you get the same schedule week after week? Per Diem means working whenever they ask you to, right? So that basically means I might work one day a month or none at all? Also, I had to select a Saturday as my clinical for next semester since my name was the last one called and that was all that was left. I won't be able to work Saturdays from May thru August, do you think that will hurt me? Thanks for answering. Sorry to sound so dumb but I've never worked in a hospital and don't know anyone who does.