How long is a day on Mercury?
It turns out the answer to this question is more complicated than you may think!
Let's start by defining that the length of a planet's day in our solar system, typically called a solar day, is the time it takes for the Sun, as seen from a given position on the surface of the planet, to move from its highest point in the sky during its day back to its highest point in the sky during the next day.
Mercury rotates very slowly around its axis (with respect to the distant stars, in about 58.6 Earth days) compared to Earth. Mercury's year (one lap around the Sun) is about 88 Earth-days long. A solar day on Mercury is actually twice as long as its year, or about 176 Earth-days long. This is because of the slow rotation of Mercury around its own axis (e.g., when compared to Earth's rotation). In fact, if you were standing on the surface of Mercury, you would see the Sun moving in strange ways -- even going eastward (or in the opposite direction to its usual westward motion) with respect to the background stars at some points of time.
To explore deeper, please see NASA/JPL's Solar System Exploration Mercury page.