when is the day as long as the night
Equinox On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes (but rarely) used to mean a day in which the durations of light and darkness are equal.[3][note 1] See Length of equinoctial day and night for further discussion.
Midnight sun At the poles themselves, the sun rises and sets only once each year on the equinox. During the six months that the sun is above the horizon, it spends the days continuously moving in circles around the observer, gradually spiralling higher and reaching its highest circuit of the sky at the summer solstice.