I never have much luck with Sunday's.  My favorite auctions are the one's that I put up on Friday for a 5 day auction and have end on a Wednesday evening.  I usually like to end my auctions about 9 pm or 9:30 PM PDT.  That way I get a good cross section of east coast, west coast and midwestern people bidding.
It also depends what you're selling.  If it's an item like a computer item where there are a lot of other auctions just like it and it's a pretty hot item, you can get away with a 3 day auction no problem.  Most auctions fall in the 5 day category.  If it's a collectible or one of a kind item, it's better to run it 7 or 10 days, so it gets the maximum exposure.  Running a hot item for 7 days doesn't do much.  You'll get action early and then it won't pick back up again until the last day or so.  
Sal