Depends on the school. If it's non-rolling admissions, I'd say schedule your interview whenever it works best with your schedule. You have plenty of time to work with before the school starts handing out acceptances.
If it's rolling... the general consensus is, "earlier is better," but I don't really think a week would make that much of a difference. If you're really concerned, maybe try looking at the school specific threads (especially from past years) to get an idea of when/how often the committee meets? I know that the adcom of a certain school meets the third Thursday of every month (or something like that), so some people were trying to make sure they had their interview done before that date.
Congrats on the interview. Good luck with whatever you decide.