Two days a year around March 21 and September 21 all places on the earth have twelve hours of daylight and twelve hours of night what is the name of these days?

Equinox.

Around March 21 is the Vernal Equinox

Around September 21 is the Autumnal Equinox

2/ [Wolf5370]

As above. Equinox is latin for "equal night" - it means the day is the same length in time as the night. The sun is on a plane with the equator at these points in time (either side of the sun).