Studies show in America that Monday and Tuesday are the biggest online shopping days of the week. In other words, people come into work from the weekend and shop at work.
I find that starting ebay auctions between 14:00 and 15:00 on Monday (for 3 day auctions) is a pretty good time slot. That is right after lunch on the west cost and towards the end of the day in the other zones not counting Hawaii and Alaska.
For 7 day, I would go start Weds because your heaviest traffic will be on Monday and Tues, and people will be ready to buy on Weds. My friend likes to do that same thing, but ends on Tues. I think either day is ok.