An equinox occurs twice a year — around 20 March (vernal/spring equinox) and 22–23 September (autumnal equinox) — when the sun crosses the celestial equator and day and night are approximately equal in duration worldwide (the word comes from the Latin aequinoctium: equal night). Technically, the equinox is the moment when the centre of the sun is exactly above the Earth's equator.
At the equinoxes, the sun rises due east and sets due west at every location on Earth (excluding polar regions). This makes them useful reference points for orienting buildings, solar panels, and understanding seasonal light changes. The vernal equinox marks the astronomical beginning of spring in the Northern Hemisphere (autumn in the Southern), while the autumnal equinox marks the start of autumn (spring in the Southern). Note that actual daylight is slightly more than 12 hours at the equinox due to atmospheric refraction and the sun's angular diameter.
For weather and climate, the equinoxes mark the transitions between the warm and cold seasons — periods when temperature gradients between the tropics and poles are changing most rapidly. This is why spring and autumn are the stormiest seasons in mid-latitudes: the increasing temperature contrast (autumn) or its rapid modulation (spring) energises depressions and frontal activity. In the Mediterranean, the autumnal equinox period coincides with maximum sea surface temperatures, setting the stage for the heaviest rainfall events (DANAs) of the year. The equinoxes also determine the timing of solar radiation changes that drive agricultural cycles worldwide.