Are stock prices likely drop off a little bit on a given friday afternoon?
It is called the Monday Effect or the Weekend Effect. There are a number of similar theories including the October Effect and January Effect. It's all pretty much bunk. If there were any truth to traders would be all over it and the resulting market forces would wipe it out. Personally, I think all technical analysis has very little value other than to fuel conversations at dinner parties about investments. You might also consider reading about Market efficiency to see further discussion about why technical approaches like this might, but probably don't work.