It would be unusual for students to be on campus (it is a school break time almost universally) and most schools depend on students as tour guides, hosts, and, in some cases, interviewers. Furthermore, as mentioned, faculty tend to take time off at that time of year so the likelihood of having a full complement of interviewers would be slim.
They're rare but they do happen. My Harvard interview was a few days before Christmas. Plane ticket was extra expensive because it was the holidays. Campus was dead. No students to lodge with so I spent extra on my own accommodations. Unless they happen in your hometown, I'd avoid interviews during this time.