Most of my sites doesn't perform well on weekends but two of them do. It depends on the content of your site.
For me, it really does depend on the site's topic. I have one site geared toward a younger, party crowd and I get almost no traffic on Friday night/Sat morning. I'd noticed the same trend on some forums in my niche, so I'm guessing everyone's out at the bars on Friday night instead of sitting in front of the computer. On the other hand, I have a couple of crafts-related sites that just kill on the weekends and a travel site that does best on Wed/Thurs. I guess that's why they say to make a lot of sites. I've finally gotten to the point where I get a pretty consistent income each day, but the split between sites is always way different.