I think for me it's a matter of week days being more regularly scheduled, and the weekend having more freedom and uncertainty. When scheduling raids with my group, we tend to do week nights and Sunday nights because that's when our schedules line up. None of us know if we're going to have any sort of social engagements on Friday or Saturday nights. And like many have said, the weekend is for catching up on things you can't do during the week - it's when I clean, grocery shop, see friends/family, and catch up on the TV shows I didn't get to watch because I was busy playing FFXIV. If I'm not doing any of those other things, then I'm playing. I've spent entire Saturdays playing FFXIV. But not every Saturday.