So this is one thing I played with over time and actually what influenced me most was other social media. I knew that if I posted my comic at 8am EST, readers would trickle in a bit slower due to simply not being awake, than if I posted at 11am EST. 11am is when twitter really starts to pick up, and at the time (and still) that's where my biggest following is so I'd post at 11, tweet my update links, and hope!
As for days of the week I've seen lots of different opinions! I update Tuesdays/Thursdays, it seems like Wednesdays are really popular days too. And though weekends seem slower on Tapas, I've heard folks say Sunday nights are great for Webtoon because that's when a lot of originals update.
Now that I only update on my website, I tend to schedule pages to post at midnight--1 because there's no trending to worry about, and 2 it's a popular time for webcomic websites to update and lots of people are still awake, watching their RSS feeds.
I'm sorry this answer is all over the place. I think it's one thing that's gonna vary depending on your audience.