Here is a graph of most of the comics I read these days...
As you can see, the largest number of updates happen on Wednesday, the same day most American comic shops have the latest issues out. The idea of new comic day being Wednesday is a relatively recent phenomenon. I can't seem to find verifiable documentation to back this up, but my recollection is that Wednesdays are a side-effect of the Heroes World collapse from 1997. My understanding is -- and if a retailer who knows this better than I do can correct me, please do so -- that when Heroes World, the comic distributor owned by Marvel, finally went under both Diamond and Capital tried vying for an exclusive deal to distribute Marvel's books. One of the reasons (though I certainly hope not the main one!) that Marvel went with Diamond was because they were willing to shift their deliveries from Tuesday to Wednesday to better accommodate Marvel's printing schedules. As the other publishers had already signed exclusive distribution deals that did not include a specification of what day books would be shipped, they were kind of stuck following whatever Marvel asked Diamond to do.
So that new comic day falls on Wednesdays is an artifact of other business decisions and pretty arbitrary. That a webcomic releases on a Wednesday schedule is at least, if not more arbitrary as they don't have any distributor to cater to. Furthermore, there's not necessarily a big overlap between pamphlet readers and webcomic readers, so why try to fit to their schedule, if that's what they're actually doing.
I mean, I get why you might not want to run a webcomic seven days a week, so you take the weekends off. That's kind of a broad cultural thing. Similarly, I can see why there might be a posting spike on Monday, as a creator might be hoping to catch people as they're starting up their week.
But the Wednesday spike doesn't really make sense to me. Anyone out there have any ideas why that might be happening?