It always depends on what you are selling and who you think is most likely to buy it.
If you are selling children's toys, you probably want the auction to end in the middle of the day for all the stay at home parents. If you are selling electronics for the 20 something male, you probably want the aution to end between 8 p.m. to 11 p.m. EST.
Also, The weekend is always a good time to end an auction, especially Sunday.