Here's a theory: They're only tracking the prostitution that is caught. (That's how crime data works, right?) Wednesday is close enough to the weekend for prostitutes to work, but there's not too many people around. On thursday/friday/saturday there are hoards of partiers & other night life so they can blend in and find johns easily. But on Wednesday they're more likely to be caught.
It's an interesting theory, but it doesn't explain why the second Wednesday has more prostitution. And following your theory, I would think that those Wednesdays have more people around because they have more money to spend.
