The day in which the length of day and night are equal everywhere on Earth is

The day in which the length of day and night are equal everywhere on Earth is called the equinox. There are two equinoxes in a year: the March or Vernal Equinox and the September or Autumnal Equinox. During the equinox, the tilt of the Earth's axis is not pointed towards or away from the sun, resulting in equal amounts of daylight and darkness at all latitudes across the Earth. This happens because the Earth's axis is tilted by 23.5 degrees and as it orbits around the sun, different parts of the Earth receive varying amounts of sunlight. Therefore, the correct option is 21st March (March Equinox) and 22nd September (September Equinox) when the day and night are approximately equal in duration all over the world.