People get interviews from November to January, with some outliers at specific programs in October and February. You can interview without using your vacation time. I saved all of mine for the end.
I take it you are nervous about the prospect of couples matching, meaning that you will likely submit more applications than you would have otherwise (unless you are planning on one geographic region). With more
submissions comes more interviews, and more traveling. Thus you may want to use your vacation time to interview.
While that is a safe strategy, most medical schools are tolerant of applicants traveling during rotations. You can't miss half a 2-week rotation and expect to get credit, nor should you schedule a required course or core rotation during your interview process. But otherwise, its ok if you take thursday and friday off to go interview in Philadelphia one week, and Friday off next to go to San Fransisco.
To simply answer your question: the two biggest interview months are December and January at least in Internal Medicine. This gives the programs an opportunity to download all the applications, weed out the nos, submit interview invites, and give time for students to respond. Most programs are also quite nice about moving you around to fit your travel schedule.
To better answer your question by giving you unasked for advice is to schedule weak electives in the time where you will be interviewing. A research elective with the PI you've already published with, for example, or a for-fun course where attendance is not mandatory. Finding those electives will not only give you the time to interview, but also the time to relax, do something simple while still getting some mental benefit, but free you from consuming all your vacation time just for interviews. This can allow you to interview as much as you need to, but still leave leisure time towards the end of your career as a student.
