Twice a year, the earth moves into a position where the sun's vertical rays are striking the equator. On those days, there is equal daylight and dark all over the planet.
March 20 is called the vernal equinox, from the Latin equi, meaning equal.
We call it the first day of spring.
And Spring arose on the garden fair,
Like the Spirit of Love felt everywhere;
And each flower and herb on Earth's dark breast
rose from the dreams of its wintry rest.
—Percy Bysshe Shelley