Today, for the Northern Hemisphere, is the Vernal Equinox, and for the Southern Hemisphere it is the Autumnal Equinox. An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor toward the Sun, and the center of the Sun is in the same plane as the Earth's equator. The term equinox can also be used to describe the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the length of the night is approximately equal to the length of the day.
At an equinox, the Sun is at one of two opposite points in the earth’s orbit where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points. In the Northern Hemisphere the days will start to be longer than the nights, brining on the northern Spring and Summer. In the Southern Hemisphere the days will grow shorter than the nights, creating the southern Autumn and Winter.
In ancient Germanic cultures this was the time of Eostre (or Ostara), the pagan goddess of the month of Ēosturmōnaþ (our April) during which feasts were held in her honor. So to the ancients, we wish for this time of year a Happy Ēosturmōnaþ (Happy Easter)!