Radiology being a regular match residency, the range of interview dates is similar to other regular match programs
-A few programs start early, with interviews invitations starting in late September for interviews as early as mid October, but since they are a few only, most students do not schedule time off for October.
-November, most interview offers are usually issued in early to mid-November after the release and review of the dean's letter. Many programs will schedule interviews mid November to mid January, with no real opportunities during the Thanksgiving, Christmas, and New Year periods.
-December, prime time for interviews through about the 20th. Last of the academic centers will review applicants and send interview offers, and most (not all!) interview offers are provided. After this, most interviews offers will be last minute, off the wait list type situations.
-January, still prime time for going to interviews. Again, you may receive invites at the last moment due to cancellations, but don't count on being able to accept them (they may be across the country with no financially and temporally reasonable way of getting there).
-A handful of programs interview into early February, including some famous, large academic programs. Most interviews in February, however, turn out to be small community programs, and this can make February important to either the FMG/IMG who needs additional interviews to have a fighting chance to match, or to the statistically challenged but aggressive candidate who also needs to pad his interview list.
So, in order of importance
-Early and mid December (Try vacation > light rotation)
-January (Vacation > light rotation > normal rotation)
-Early to mid November (Probably won't need vacation, but stay close to the computer for invites)
-October and February (Just make sure you can take a day off when you need to).
Remember that you will need time to interview at enough prelims/TYs, so the real interview list for most people going into rads is anywhere from a total of 10 to 40 total interviews, so it can get intense when scheduling. I know of people on the interview trail uninterruptedly for 2 weeks to >1 month.
Hope this helps
Also depends on where your applications are focused on around the country. Midwest, Texas, Arizona, and Southeast tend to send out interview invitations earlier, thus offering interviews in Nov-Jan. Almost all of California, I believe a lot of the northeast, and almost all big name places send out invitations later in the process. (Wake and Virginia follow the southeast trend, Duke does not). So interviews here in November are not going to happen because they haven't even given out interviews yet, these will primarily be in January, maybe into Febuary.
If your applications go all over the country, ideally I would try to be free for anything November through January. You don't need October free and you don't need Febuary free either. I fit 18 interviews in Nov-Jan, without a problem. Conflicts in dates are not that frequent. Also, when you schedule interviews, three in one week was exhausting particularly if you have any other responsibilities and do it in back to back weeks, two a week is perfect if you can schedule it this way.