I'd *STRONGLY* recommend buying a calendar and listing the possible interview dates of your sites. Not all of these dates will be listed, but many will be available in the site materials. Don't freak out if you don't know when some sites offer interviews, but the more dates you have, the better informed decision you can make about interview date selection.
It will be very helpful to know which sites may have conflicting dates and which sites will have unique interview days. For applicants who receive 7+ interviews, it is particularly helpful because many sites will have MON & FRI dates, and there are only so many Mondays and Fridays in December and January. As a general rule, if your site interviews mid-week, I would recommend taking that date if you can swing it. Less sites offer mid-week interviews, so you will be less likely to have conflicting dates. Mid-week dates can also help save on travel if you can fly destination to destination because the cost of 1 night at a hotel is typically less than a RT flight and less wear and tear on the interviewer.