Not being a mailer, but having some experience with newsletters, I'd suggest mailing Tuesday night or Wednesday morning.
Everyone is inundated with spam in their inbox on Monday morning from the weekend, so the first thing they do when they check their email is delete all their spam. You'd probably get lost in that crowd.
Friday's too close to the weekend and the weekend isn't any good imo....especially in the summer time.
Not an expert on the subject, just my
