It seems that every time our piano tuner comes to our house, he scolds me for waiting too long between tunings. I agree with him that it should be done every six months, but I don't really think about it until the piano sounds off-key. Last time he came over, I was on the defensive.
"If you would send out a postcard reminder like the dentist," I declared, "I would make sure to call you for an appointment in a timely fashion."
Without hesitating, he replied, "From now on, when the dentist sends you a postcard, call me."
WHY YOU NEED TO TUNE YOUR PIANO
Tuning a piano is a process of adjusting the tension of each piano string using a tuning hammer to turn the tuning pins, so that the pitch of each string sounds pleasingly in harmony with every other string according to certain known acoustical laws and aesthetic rules and customs. If your piano is not properly tuned, it's acoustic performance will be degraded, therefore tuning a piano regularly is a necessary maintenance.
There's an ongoing debate as to how frequently a piano should be tuned. A good rule of thumb for the average household in a moderate climate is to tune the piano when the heat goes on in the Fall, and again when the heat goes off in Spring. Piano manufacturers suggest tuning two to four times a year. In my experience, the average piano owner gets the piano tuned at least once a year, although it is highly recommended it gets tuned twice a year.
Whether a piano gets played or not, it will go out of tune due to it's environmental changes. It is not a good idea to wait until you begin hearing 'sour notes'.
If a piano has not been tuned for quite some time and sounds badly out of tune, it probably needs to have 2 or more tunings in order to bring it back up to pitch.
HOW YOUR PIANO GETS TUNED
There are about two hundred strings in a piano that are stretched across a cast-iron frame, one end of each string being attached to a hitch pin and the other end coiled around a tuning pin. The pitch of each string when vibrating depends, among other things, on the tension at which it's stretched. By turning the tuning pin, the tension can be tightened or slackened, and thus the pitch altered. Tuning, properly speaking, is only the operation defined above, and does not include repairs and adjustments, fixing squeaky pedals, cleaning, and so on, as is often thought to be the case.