Parts of the trading day
The most volatile part of the trading day is the open. Most traders consider the opening volatility to last roughly a half hour. Some daytraders will wait until after 10:00 before initiating positions because the intraday trends haven't revealed themselves yet. Some swing traders will also ignore the first half hour because the intraday volatility trows them off. I have done this in the past. There were times when I had swing trade positions in stocks which I knew would be volatile at the open and I didn't want to get scared out of my positions if the opening volatility was high so I would not start watching the market until 10:00. I don't do this anymore as there are too many good opportunities at the open and I am now much better at handling intraday price movements - although there will be times where I may not watch the market at all if I want to ignore intraday movements.
The closing half hour can also be very volatile although it is not anywhere near as volatile as the open because the open has built up supply and demand from the market not being open for the previous 17 hours. The time when the market gets volatile on the close happens most often when the market has been down alot lately - like when the Dow drops 500 to 1,000 points in a week or two. In these cases you'll see days where the Dow is up a decent amount - like 50 points or so - during the day but it seems like traders are afraid the market is going to tank. It isn't until the last hour, and especially last half-hour, where people realize the market won't be dropping where you'll witness the Dow going from +50 to +120 between 3:00 to 4:00.