Mercury is the easiest planet to find but the hardest to see. Look up at the sun and you are also looking at Mercury since it is never far from the sun in our sky. Of course Mercury can't be seen (naked-eye) during the day but it can't be seen at night either since it rises and sets so close to the sun.
So when can Mercury be viewed? In twilight. As it orbits the sun, it moves in our sky from one side of the sun to the other. When it's close to maximum elongation (angular distance from the sun in our sky), Mercury may be viewed in twilight before sunrise or after sunset. These periods don't last long and a clear view of the horizon is usually needed. Mercury goes through phases and a telescope can reveal the current phase.