equinox

päiväntasaus

päiväntasauspiste puhekieltä The intersection of the apparent path of the sun in the sky (the ecliptic) with the celestial equator.
One of the two days on which this intersection occurs each year: (for the Northern hemisphere) March 20 or 21 in the spring and September 22 or 23 in the autumn.
2005, Clive L. N. Ruggles, Ancient Astronomy: An Encyclopedia of Cosmologies and Myth

Since it gets light before the sun rises and remains light after the sun sets, the actual period of darkness at the equinox will be substantially less than twelve hours, the exact amount depending on latitude and how one defines the boundary between twilight and night.