CASE started extending interview invitations in June! So, I'm sure that they've been systematically filling up the interview schedule. Most likely, December 1 is just the next date that's available. They do around 350 interviews, and they only interview M,W,TH,F. At each interview they have around 5 interviewees. So that's about 20 interviews a week. The first interview was Aug. 16. Making a crude estimate, there are about 15 weeks between Aug. 16 and Dec. 1. That's about 300 interviews. Sounds like you grabbed one of their last interview spots!