Diagram 3: Days and Years on Mercury
Mercury rotates exactly 1.5 times in each revolution.  Let's start at local noon on our reference meridian (north south line).  We can see that in the time it takes for Mercury to rotate one quarter rotation (90o) it has revolved one sixth revolution (60o).  After one half revolution, three quarters rotation, we have sunset.  At the end of exactly one revolution (year) it is midnight.  After 1.5 revolutions, 2.25 rotations, we have sunrise.  At the end of exactly 2 revolutions (years) which is exactly 3 rotations, it is again noon, completing exactly one solar day.  One mercurial day is two Mercurial years long.  Since one Mercurial year is about 88 Earth days long, one mercurial day is about 176 Earth days long.  By far the longest day of any planet in this stellar system.

(1.5 rotations / 1 revolution) - 1 = 0.5 days per year