I help run a .co.uk auction site for PCs and our webtrends repeatedly show that you get the most hits on a Monday and the fewest on Saturday/Sunday.
The busiest times (on average) are between 16:00 and 18:00. Most listings are timed to close between 16:30 and 17:30 but if there's activity in the last 10mins then it's extended by a further 10mins etc.