Hi, last year all of the invites went out on a Friday and Saturday for the following week. I don't think the invites went out in waves because we all had our offers by Valentine's Day. I got my invite on a Friday and my interview was Monday. It was with the concentration chair and my would be (current) advisor.
I was asked questions like why I applied for an Ed PhD program (I earned my MS in a discipline), my advisor asked me very detailed questions about methods (but I think that was specific to her), and I think they asked me a question about my career aspirations...
I was honestly very long winded; therefore, they were only able to ask me like 3-4 questions. The interview was on the formal side.
If I were you, I would definitely be able to articulate my research interests (and how those interest fit with the concentration you are applying for), why I'm interested in an education PhD program, and how I plan to use research to impact change in education.
ETA: Also Good luck! I'm excited for you (I remember that feeling last year).