From an index (S&P/Nasdaq/Dow) perspective, classic wisdom says the average day goes like this:
9:30 to 10:00 am - trading range
10:00 to Noon - higher volume part of the day - counter trends
Noon to 1:00 pm - lowvolume..naptime
1:00 pm to 4:00 pm - more of a trend, but lower volume...