I have been day trading for a month, and I've noticed that the first twenty and final thirty minutes of the trading day seem to have repeating patterns. At the beginning of the day prices will often move to one extreme on high volume, then bounce back starting around 9:50. The 3:30 ramp is another well known phenomenon, affecting the S&P as well as other stocks on occasion.
Are these the times that large institutions do their trading? What forces drive these patterns?