Hi Adam, from a personal preference, I would prefer beginning of the week. I carve out the 1st hour of Monday to review data from last week, catch up on email, etc. Maybe consider sending Sunday morning in case those activities are done by your customers on Sunday. From a customer standpoint, my team gets more responses to updates going out Monday/Tuesday than Friday.
Hope that helps, would be interested to see what your A/B testing shows when completed. Brian