Sundays are actually one of my better days on average. Tuesday is the best, then Monday, Friday/Saturday the worst.
My own sites seem to jump in revenue from Adsense at the weekends. Starting Fridays and through Saturday I usually get double the money of other days of the week.
for me weekend traffic has always been low (adsense earnings are directly proportional) but on the flip site tuesday and wednesday around 9am GMT are the best
I see Adsense on 'relaxed' sites increasing over weekends (travel, reading, celeb etc sites), but business related sites do worse. If your market is office bound employees, you'll see a decrease during the weekends as they're accessing from work...
yeah, it definitely depends on the topic of your sites as you pointed out. I own a pretty "relaxed" network of sites, mainly gaming and earning are definitely about half a much as they are on work days, due to schools not being in session, people not slacking at work etc. And this weekend was by far the worst weekend in a while, lowest CPM rate day on Saturday since September, very discouraging when your traffic grows and your profits shrink.