When Should I Have My Piano Tuned?
When to tune your piano depends on your local climate. You should avoid times of rapid humidity change and seek times when the humidity will be stable for a reasonable length of time. Turning the heat on in the house in the fall, and then off again in the spring, causes major indoor humidity changes, and in each case it may take several months before the piano's soundboard fully restabilizes at the new humidity level.
In Boston, for example, the tuning cycle goes something like that shown in the graph. A piano tuned in April or May, when the heat is turned off, will probably be out of tune by late June. If it is tuned in late June or July, it may well hold its tune until October or later, depending on when the heat is turned on for the winter. If the piano is tuned right after the heat is turned on, however, say in October or November, it will almost certainly be out of tune by Christmas. But if you wait until after the holidays (and, of course, everyone wants it tuned for the holidays), it will probably hold pretty well until April or even May. In my experience, most problems with pianos in good condition that "don't hold their tune" are caused by poor timing of the tuning with the seasonal changes.
Note that those who live in a climate like Boston's and have their piano tuned twice a year will probably also notice two times during the year when the piano sounds out of tune but when, for the above reason, it should probably not be tuned. The only remedies for this dilemma are to have the piano tuned more frequently, or to more closely control the humidity.