For my team it's all about being consistent and here's our rules for it.
Rule 1 - Stay consistent
It honestly doesn't matter what day you choose to post. You're going to be competing with whatever else is around your readers anyway. Loyal readers really don't care when a post is made, as long as they know it'll be there consistently. I have only seen one or two webcomics do well when they haven't had a consistent posting schedule and they are seriously the exception to that rule. Pick a day in the week, or a day in the month and stick to it.
Rule 2 - NEVER miss a post
This doesn't mean your post has to be AN ENTIRE PAGE. Sometimes the most stupid page you throw up on the comic will have the biggest effect. I have a friend who has a comic that she ran, and she'd had nothing to post that week. So she just threw together a black box, and the word 'uhoh' in a bubble and put a clever tagline about "What you don't want to hear in bed at night," or something. It took her literally five minutes. She posted it, and got to work. When she came back? The stupid thing had gone viral. It was one of the LAZIEST thing's she'd ever made and people went nuts over it.
Even with narrative comics there's a way to do this. The comic I work on, Kamikaze, was in dire straights one week. A convention had come up, we were working all weekend, and by our update day had next to NOTHING to show for the week. So we used what assets we had, threw together a background and came up with this page, which was an emergency bulletin that fell in line with the story. People LOVE that page. It's simple and easy and it took us no time at all to make.
Your readers loyalty will continue to show up as long as they know that they'll see something each update. Don't let their attention wane, by giving them the disappointment of a missed post.