The end time of auctions is very important.
For instance my help children sleep CDs sell very badly if they end between 3-5 pm simply because the parent who might buy such a CD is going to school to collect their child.
Most auctions end between 8pm-10pm however if you are selling in a very busy category you might find selling at this time is of detriment to you and that you do better selling say, at 1-2 pm.
A lot of people who sell are at work and as such, lunchtime is an ideal time for them to surf the Internet.
The best results for your product can be obtained by viewing the closing price of competitors items, this can be done through ebays advanced search on items that have already sold.
Sometimes you find the strangest times are the best time so try and keep open minded, sometimes seasons also have a part to play. For my own products, weekends are a quieter time, Sunday the slowest day of them all.
Many Powersellers also find the end of the month slower, and the beginning of the month the best time of all as that is when most people are paid.
And then to make life even more confusing you might be selling in another country all together, it really is a lot of trial and error.