Pre-Market/After Hours : This is the Grey Zone: Very Little Liquidity, Massive Position Altering Moves often occur in the grey hours.
Initial Move: Stocks usually get a big thrust first thing in the morning as Buy/Sell orders flood the tape
Secondary Move: Often times there is some sort of reversal as Overnight Speculators take profit on Winning Positions
Mid-Day Chop: Alot of the liquidity from the morning tends to dry up in the middle of the day. Alot of traders take the middle of the day off and eat lunch. Stocks like to chop around in the middle of the day and also run stops. Trading during this time period tends to be slow, and prone to whipsaws.
Power Hour: Trading begins to pick up again as the end of the day nears and people begin re-positioning. Oftentimes but not always the direction that it closes into will presage a gap in that direction the next morning.
Final Minutes: Just like procrastination in life: Many people wait until the bell is about to ring to alter positions.
Knowing where you are in the trading day can help you make better trading decisions.