I always prefer that mailings go out Tuesday – Thursday.
Monday is not good because open rates are at their lowest. People have tons of email in their inbox and delete everything that even remotely looks like a sales offer. They also tend to be cranky on mondays.
Friday tends to find people distracted and eager to get done with things and out partying. They are less receptive to offers and even if they open email they are heading out and away from their computers so they dont play.
These are my observations. Other affiliates may have wildly different ideas and experiences.