I don't think it really matters WHEN you post your comic, just as long as you're consistent with it. When you're just starting out a comic, it's going to take a few updates for people to even notice or get interested. That's not to say "omg no 1 will luv u" or anything so silly, it's just that it takes time for readers to know what to expect and when to expect it.
Keep it coming on a specific day, at a specific time, and stay as consistent as possible. Even if you don't have anything ready just put SOMETHING up . It can be a drawing or a bunch of sketches, or a super simple doodle to hold the page until the next image is ready. If you keep new content coming, no matter how simple, your readers will thank you for it.
-C