Most do have them on Fridays and/or Saturdays. But I suppose you never know. If you get an interview offer, it will be several weeks in advance of the interview so you will have plenty of time to plan your schedule accordingly.
I had one that was on a Monday. The other three have all been Friday. Although one school had the option to do Saturday instead. I'm glad I chose the Friday date, because we only had 30 people there, and they said their Saturday date had 60 people.