When I interviewed, the interview options were generally Thursday, Friday, or Monday for my school. Some people received interview invites as close to the date as the week of the next interview, although it is a rolling admissions school, so they had the option of picking later dates when they opened.
I had to get the date off work, but I didn't have any classes. Many students said their professors were understanding, but a couple were balancing which classes they couldn't make up with which interviews they'd attend. I believe one person told me he was going to take the C in a class he previously had an A in so he could attend interviews. I think it's best to be up front, give as much notice as you can, and make sure the prof has a positive opinion of your work ethic otherwise that semester.
