This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
Again, it depends on what you sell. Ebay seems busiest on Sunday nights, but
there are numerous reasons why ending auctions at that time may not be a
good idea. For instance, if many of your competitors are ending auctions at
the same time, the bids may be split between those competitors and the final
values might be lower. Also, keep in mind that holidays are generally a bad
time to end auctions.
Also, consider your audience. Some items sell very well during the daytime
since the bidders for those items are surfing from work or when the kids are
at school. Other items sell better with evening closing times for similar
reasons. Experimentation is strongly recommended. And don't forget that
you'll need to retest your assumptions periodically since buying habits
change with time as well.