Most medical school, residency and fellowship applicants schedule their interviews according to convenience and cost, but you may be asked why you arranged your interview so early or so late in the season. The real concern behind this question is that you are interviewing early because you are using the institution for a practice session or that you are interviewing late because you had considered leaving the institution to the end in case you wanted to cancel.
Reassure the interviewer that you are serious about the program. If you are interviewing early you might say, "As soon as I was offered an invitation I took the opportunity to visit because I am very enthused about this institution." If late, you might say, "I wanted to maximally prepare for this interview because I am very interested in being here."
Contact me soon for mock interview assistance. I am scheduling two to three weeks in advance.