Monday to Friday between midday and 4pm typically works best, but ultimately the buyer is most likely a reseller, so if they want it, they'll find time. If there is a potential of end-user bids, then certainly having it conclude during the week is important. Consider if there is a late bidding war and how that will impact the end time...I once had one buyer stop bidding because he had to go pick his kid up from school, otherwise would have paid much more.