The only school that I have heard of that interviews applicants on weekends is UTSW. For the others, think of it this way: an interview day is a huge thing to coordinate -- what interviewers and admissions coordinators want to work on weekends in addition to the usual M-F? Not many. Don't worry, I was under the same impression when I started interviewing last year (although I had school and not a job to worry about). In fact, many schools do not just let you randomly choose a date; they have set interview days and you usually get assigned one. However, these schools are very good about letting you move your interview date--just call or email. Also, if you have some interviews in the same geographical location, you could do them together. I found that some schools (like Cornell, for example) gave me an interview invite right after I emailed them to tell them that I would be in the area for a Columbia (or any other NYC) interview. Good luck to you in the process!