Unless things have drastically changed in the Admissions Office since I was an applicant, you can expect Penn to be shooting out interview requests pretty regularly for at least the next two months. The school doesn't try to fill their class quickly like many other schools. My interview was in February and a few of my classmates had even later interviews.
Interviewed December 5th and accepted the 12th via snail mail!
Don't get your hopes up, there are a lot of late interviews, and if you already got an interview and waiting on a response, the holidays is here so I would think they're off around this time. Good luck!