At exactly 2:17pm on Friday. Never an exception. "They" got together and decided on that time long ago. Easy money can be made--just sell an option at 2:16 and buy it back at 2:18. Or maybe the market isn't stupid enough to do it instantly and instead depresses IVs slowly throughout Thursday and Friday. Nah, it's free money.
Does the short side have an edge if you short before the close on Wed and hold it for the weekend, assuming the underlying trades flat?
Unfortunately not. By the time Monday rolls around, the option has been discounted an appropriate amount.