I personally chose Thursday. People say it's good ti ein the middle of the week. The old version was posted on wednesday, but we go shopping on that day so that was not a good idea :,D
I don't know how much hours matter. They practically affect only people in your timezone. When I update, I know more people will come over time, just because they are asleep at that moment, at work etc. I post 2:00 PM of Central Europe time. I strongly recommend to make multiple tweets and social media announcements, just to be sure even people in different timezones get to it.