Why Does a Piano Go Out of Tune

The most important factor that causes a piano to go out of tune is humidity changes.

A piano goes flat in the early winter when the dry heat of the furnace draws moisture out of the soundboard, diminishing the crown. It goes sharp again in the spring when you turn the furnace off for the season and moisture from rain enters the soundboard, increasing the crown.

Fluctuations in room temperature surrounding a piano cause less of a change in tuning than humidity changes do.

The louder and more often you play a piano, the faster it goes out of tune by a small amount. This is due to equalization of tension along the length of the strings.

So, every piano is subject to one or more factors that will make it go out of tune, including humidity and temperature changes, stretching of the strings, slipping of the tuning pins.

How often should you tune a piano?
This depends upon its condition, the environment in which it is located, and the musical demands of the piano player.
A piano that is played used regularly and is in good condition, might get by with being tuned twice a year, each time the seasonal humidity changes.