penn is non-rolling i believe. they won't have made their decisions by then, so you're not interviewing for the waitlist.
january interviews are fine, feb too i'm sure. march or later is when i'm concerned about interviewing for a waitlist spot.
The medical school I eventually ended up at, I interviewed in late January.
Even if you got an interview in late April, the school that you're interviewing at may have not filled their spots completely. It's just alot harder because you're competing against so many more people for those few spots, especially if the school has rolling admissions. (Bascially you're competing against everyone that interviewed before you that was not accepted for one reason or another.) If you interview late and "wow" them, they'll take you.
