Don't know, probably evening because if people are selling stuff they will want it to end when people are at home rather than at work. Just a guess though !
It depends on what you are selling - think about your target buyers.
Young clubbers? Busy mums? OAPs? Workers?
It doesn't matter to me what time an auction ends - if I see something I want, I put my bid on using JustSnipe. That puts the bid on for me a few seconds before the auction ends.
I reckon early evening. Sometimes the site's very slow and unresponsive between say six and seven. I think it's people logging on when they get home from work.