Equinox is a word coming from Latin. On the date of the equinoxes, the lengths of the day and that of the night are the same (or equal). In other words, around the day of the equinox, the length of daylight being present is the same as that of the night. This needs to be measured on flat land or the ocean, because any mountain or hill will be an obstacle.
The length of daylight is quite easy to measure. This was done in most cultures. Usually, festivals were celebrated at the equinoxes.