Preliminary interviews can be as early as September, depending on the program and the applicant. The interviews continue through January. There will be instances when a pre-lim interview and a rad onc interview are on the same day. As you can imagine, it's not a tough decision to make regarding which interview to schedule. There will be plenty of pre-lim interview dates, but only a few rad onc interview dates per program.
Even if you miss out on interviewing for a pre-lim program altogether, always take the rad onc interview.