Vernal Equinox day

Why is it called equinox?
On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night".

However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight

The March equinox occurs the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens either on March 19, 20 or 21 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis doesn't tilt neither away from nor towards the Sun, like the illustration shows.
( from timeanddate.com)