Mercury's rotational axis isn't inclined, the way Earth's is. So it has no seasons. Which (as far as I can tell) means that days are the same length no matter where you are. (Too bad; I was hoping for a land of perpetual twilight and midnight sun up by the north pole.)
Mercury's days ARE 58 earth days and change long, though. Unless the Silver Millennialists decided to modify their planet or something. Or the GM makes changes for dramatic effect.
As a side, for another possible dramatic effect, the terminator line that divides night from day sweeps across Earth at about 1670 km/hour, so we don't really notice. But on Mercury, if I've calculated correctly, it only travels at 10.89 km/hour.
Er, that bit about the terminator speed? That's at the equator, since it depends on the circumference. (Essentially, take the circumference, and divide it by the length of the day.) If you're standing on the rotational north pole, then technically there's a circumference of 0, so you'd be in constant sunlight.
Hm. This also seems to mean that the days get shorter as you go further from the equator. After all, the angular velocity is the same no matter where you're standing, but the radius from the rotational axis (and thus, circumference perpendicular to the axis) is getting smaller.
Either that, or I need to get more sleep before I try to do physics in my head. It's 2:00 AM right now. Goodnight.