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Mercury revolves around the sun very quickly, but rotates around its axis very, very slowly. One day on Mercury (sunrise to sunrise) is longer than one year on Mercury (one orbit around the Sun).
Mercurian Year
A year on Mercury takes 87.97 Earth days; it takes 87.97 Earth days for Mercury to orbit the sun once.
Mercurian Sidereal Day
Each sidereal day on Mercury takes 58.65 Earth days; it takes Mercury 58.65 days (2/3’s of its year) to rotate around its axis once. One day on Mercury seems to last two Mercurian years (or 176 Earth days), i.e., sunrise to sunrise. It is daytime for one Mercurian year, and nighttime for one Mercurian year. (It used to be thought that Mercury always kept the same side side towards the sun, but this is not true.)
These figures show the path of Mercury orbiting around the Sun. The red marks indicate the same spot on the surface of Mercury at different times in its orbit.
A point on Mercury that is directly facing the Sun will point in the same direction after one rotation (59 days = 2/3 of Mercury’s orbital period), but that point will no longer be facing the Sun. That point will return to the same position in three rotations of the planet during which it will orbit the sun twice (or 176 Earth days).