When is the best day/time of the week to release a new issue?

I would take into consideration what your primary audience is and where they are. Typically for social media, 11am to 5pm local time (or wherever your audience is) is considered the "best time". No matter when you post though, I'd make sure that you have a set of promotional posts ready to go out.

I tend to post on the traditional new comic day of Wednesday. Wether that is the right decision or not remains to be seen but as @ArtCrumbs has hinted at, I think the level of success you are likely to achieve has as much to do with the work you put into promotion more so than the time/day it is released.