How often should a piano be tuned?
In New England, twice yearly is recommended: 1) after the furnace goes on in the late fall, 2) once the furnace is off and humidity has risen in the early summer.

Why do pianos go out of tune?
Anything that will change the tension in the strings will potentially cause the piano to go out of tune. Humidity and temperature are the usual culprits. The soundboard is a thin piece of wood which will swell or contract depending on humidity and temperature. When the soundboard swells (in the humid summers of New England), the strings are stretched tighter and the piano goes sharp. When the soundboard contracts (in the dry winter when the furnace is on), the strings go slack and the piano goes flat. Heavy use can also alter the tension in the strings, e.g. practice room pianos are at risk for going out of tune more frequently than most.

How is a piano tuned?
Using a tuning hammer, the pins are turned to tighten or loosen the piano's strings. In this way, the tension of each piano string is adjusted so the tones produced conform to a specified interval. When the octave is divided into 12 equal intervals, the tuning is known as an "equal temperament". Equal temperament tunings are the most commonly used tunings for pianos.