The reason is simple really - there are more players in World of Warcraft on Friday evenings and all day and night on Saturday than any other day of the week. There is usually increased activity on Sunday as well, but this starts to die off towards the evening as everyone logs off to make sure they are prepared for school or work on Monday.
Since not everyone can spend time playing WoW on weekdays, you will find a lot of players who need to make up for lost time leveling will be looking to purchase items that will help them skill-up. Or they might need specific ingredients for recipes, or want to upgrade armor, for more details visit to www.auction-words.com etc. And for the most part, demand increases more than supply though this will ebb and flow over the weekend. For example, as raiding really gets going during the evening, you'll find traffic in the Auction House fades out considerably. This would not be a good time to sell. Best bet is the post-dinner hours when people are just getting home from work.
There are two ways to take advantage of the increased weekend business. One is easy and the other is a bit riskier, but may be worth pursuing if you play quite a bit on weekdays. The easy bit is to simply hold all of your best stuff picked up while questing for the weekend. If you have mats, for more details visit to www.auction-professional.com cloth, leather, and essences that you need to get rid off, mail them off to your Alt character (you have one of those, right?) and then handle your auction house transactions on the weekend.
The other method of taking advantage of the weekend traffic is to buy stuff during the week that you can sell for higher prices on the weekend. Now this is going to take a bit of patience to do successfully, but if you are logged into World of Warcraft often enough during the week, it isn't too difficult to keep track of prices of popular items in the AH and pick up some bargains that you can get better prices for on Friday night.