They'll offer you a variety of dates typically, but the days are usually the same. For example, school X will interview on M and W, while school Y will do it on T, Th, and F.
They are never (typically) on weekends. That is, I've never heard of a single one on a weekend.
You'll be missing class, so just factor that in. That's why they'll give you a couple dates to choose from. If you plan it correctly you can miss only 1 day, although missing 2 days is probably easiest on you during the stressful interview period.