The word 'equinox' means 'equal night' and is often seen as the days when day and night are each exactly 12 hours long.
But that is wrong: such days are 'equiluxes' (equal light) and occur a few days before the equinoxes.
The reason is that daylight is from the first appearance of the Sun over the horizon to the end of its setting.
On the equinox it is the centre of the Sun that is visible for exactly 12 hours.
The equinoxes are the only days when the Sun rises directly due east and sets due west in the northern hemisphere.
An old myth was that eggs could be stood on their sharp ends only at the equinox. Supposedly that was because the gravitational pull of the Sun directly overhead exactly balanced that of the Earth. That is absolute rubbish.