What I wanted to share was the fact that there was a noticeable difference to begin with, so it's something worth paying attention to.

The exact pattern will depend on the audience. If it's VmWare emailing IT people, then the open rate is probably going to be the same through the workdays and zero on weekends. If it's some hobby stuff, then it'd be in reverse. For my particular project, it just happens that Saturday sucked, which is a good thing to know.