It depends on the school. I had 4 interviews, 2 of them were midweek and the other 2 were on Saturdays.
Sometimes schools will give you a list of interview dates when they invite you so you can pick one that fits best with your schedule. You can usually schedule it to only miss a day of class so it isn't a huge deal depending on what classes you are taking.
I went on 4 interviews and they were all during the week. Most schools give you a few days to choose among, but I think as the interviews get closer to the end, they pick your day for you and if you can't go, you're SOL.
If you can schedule your semester so that you're off on friday it might benefit you I had a few interviews on friday, if you cant you cant and you just miss class