Research shows that the best day to gain an increased response is a Tuesday / Wednesday. Always send out your emails on Wednesday between 2-3pm and make sure its on time every time. (Yours subscribers will be waiting, don’t let them down)
Mondays are for recovering from the weekend and Thursday and Fridays are for planning the weekend. Not a good idea to send on these days I guess.
But why not ask your subscribers when they prefer to receive their emails, I am sure thats the best way to find out.