Seasonal changes cause expansion and contraction of the [pin block] and all the wooden parts of the [piano action], (the mechanical part of it). The effect of seasonal changes on the ‘pin block’ may affect the tightness of the [tuning pins], and thus can result in a variation of the string's tension, causing it to go out of tune…
Although most modern houses have a regulated heating system, (by opposition, wood stove can be a problem), seasonal changes do not cause too much change in a piano tuning for most of the year. Except for the summer time (4 months for most of Vancouver Island) when outside temperature rise above the commonly comfortable house requirement of 21°C.
Temperature and Humidity
Temperature and humidity play a very large role in your piano's health. Although a small variation of temperature does not cause a perceptible change in a piano string's pitch, frequent changes, (such as a wood stove as a primary source of heat in a house), may significantly affect the overall tuning.
On the other hand, humidity affects both the strings and the wooden parts in your piano. Strings exposed to humidity will tend to create a layer of rust on its surface changing the resonance of the strings. (and colour turning brown).
Effects on Wood
Wood expands and shrinks with humidity variance. The [tuning pin], which the strings are attached to, are embedded in a wooden [pin block] under pressure. So as the wood expands or contracts, it will alter the force that squeezes the pin in place against the pull of the string. Hence, potentially changing the tuning.
The [sound board] is a beautiful thing. But drastic fluctuation of temperature and humidity could cause cracks in the sound board. A seriously cracked sound board could cause a "buzzing" noise when playing. Some repair may be executed, but do not expect a full recovery to the original sound of your piano.