good question..... I like to list my stuff on Sundays with the auctions closing between 6pm - 9pm eastern time. It gives the buyer all week to look at it, and most people will be home at that time... since they usually have to work the next day. Auctions closing on Friday or Sat night get less sniper bids since a lot of people are out doing stuff.
Also, I like to post auctions which end after the 15th and 30th of each month... when most people get paid.
I have no proof that this actually increases closing costs, but I figure, people are more likely to spend money when they just got paid.