It gets really hot (430°C, or hotter then an oven) during the 3 month long "day" and −180°C during the equally long "night". This is caused by its lack of any real thick atmosphere, so nothing to keep in heat.
(Mercury Rotates on its axis in 58.6 days. This is 2/3 of its revolution period and is an example of a tidal lock or resonance. After two revolutions or three rotations, the same face of Mercury faces the Sun. (176 days) 
Mercury is actually the smallest of the planets in the Solar System as Pluto is no longer considered a planet. Exoplanets as small as Mercury haver not been discovered yet so Mercury is the smallest planet known.