From Penn:
Question: Is there anyway to know when the application reviews and interview invites will be sent out by programs?
Answer:
There is no set schedule for invite notifications—invites are managed by each group individually in advance of their interview weekends. As noted below, some groups do make their interview information public, including CAMB.
Applicants are accepted on a rolling basis between late January and April 15. All final decisions will be available from BGS by April 15, though the vast majority of applicants will be notified well in advance of this date.