People are gonna tell you Mondays are the best day, but all of the stats tracking I've done tells me that basically any weekday is the same as any other.
Sundays and Saturdays see a bit less traffic overall, but usually no one updates on those days so there's no new content for a visitor to check anyway. Besides, they'll just wait until a weekday.
So, I'd suggest that it doesn't really matter which day you update. The most important thing is to be consistent. If you pick a day you should always update on that day. Your readers will visit on a day of the week that works for them and if you're updating, they'll have something new to see, which will make sure they come again in a week
As long as you pick a day that works for you and get word out so people will look at it, you should be fine. There's a few weekly comics I read that update on Sunday of all days and they seem to be doing just fine (sometimes it's nice to have something new to read on the weekend).
Well, I think you should pick a day (or days) that are either most convenient to you *But make sure you always update that day, or days*
But if you update twice a week, pick days that are farthest away from each other. It's usually a good idea, because if gives you time to prepare for them.
Not necessarily what you have to do, just a thought.
By the way, consider making a buffer or at least uploading ahead of time. THen just set for when you want it to post. That way you don't have to worry about making sure you upload on the same day, just whenever its convenient.
I think the pattern of views will tend to change for every comic out there. For me I tend to have a lot of readers who just check in once a week, despite my update schedule being Monday through Friday. So basically, as skool said, your readers will check in on whatever day happens to be most convenient for them so as long as you update consistently, it won't matter in the slightest what day it is that you post the page.
Now as for time of day, most people seem to swear by midday since it gets you a bit of front page time. I don't really know that it makes a difference though. Most of the more popular comics on the duck tend to update at midnight and I don't think it affects things much.
For me, any day but Sunday - I've noticed that my traffic is always lowest on Sundays. None of my favourite comics update on Sundays either, and overall, this place seems to slow down then.
But as long as you're active on the forum and people notice you, it won't really matter what day you update.