During the summer, the ceiling fan should blow in a counter-clockwise direction, and during the winter, it should blow clockwise. One direction blows air straight down and the other raises air toward the ceiling.
During the summer months, switching the ceiling fan to blow air straight down will help to quickly cool a room and keep it cool. Looking up at the fan, it should be running in a counter-clockwise direction during warm weather.
During the winter, turning a ceiling fan to blow in a clockwise direction will draw air toward the ceiling. This forces warm air down, gently warming the room.