When I got the interview invite I had this exact same question, so I called to ask. The lady that I spoke to on the phone said that they just condense everything down if you interview during the week. From the way it looks on the interview schedule, it looks like there is a lot of wasted time on the weekend interviews (organizational meeting, 3 meals...???). But for me, being able to interview on the weekend allows me to only miss one day of school instead of 2+ days.