A mercury day if you consider its solar day is 2 mercury years.
Now if you consider just 1 half of mercury, in 1 of those years it is facing towards the sun and in another one of those years it is facing away from the sun.
Mercury is tidally locked. This tidal locking happens when the rotation rate slows down until it reaches 0.
How is it possible for a planet to not rotate and yet have 1 half of it facing towards the sun each year with that half changing every year? It would seem like it would have to rotate 180 degrees in a fraction of a second which I know is not possible.