The day really depends on how many days your auction lasts.
The time is different. You don't want people to be sleeping or having lunch/dinner/breakfast when your auction is ending. So, to avoid lunch, the best time is probably 1-2pm (PST), you'll get most of the people on the West Coast and Central, and some of the people in the East Coast. At night, you might want to try 7-8pm PST but so that you get people in the West and East Coast and everybody in between.
Hope this helps.