In layman's terms, the equinox happens because the planet earth is not tilted towards the sun while the sun is directly above the equator. This causes the equinox to happen.

So during this time of the year, both hemispheres get the same hours of daylight and night.

On a usual basis, the earth is tilted as it orbits around the sun, about 23.5 degrees. This tilt causes the difference in how much of the sun's radiation we receive at various times of the year.

The earth's year is not a consistent 365 days, often there is an extra quarter of a day that accumulates each year which causes the date of the equinox to shift.

Nevertheless, the equinox usually happens to mark the astronomical beginning of spring or autumn, but the meteorological beginning of these seasons are March 1st and September 1st.

This 2021, the spring equinox is bound to happen on March 20th, and the fall equinox to happen on September 22nd.