In astronomy, the vernal equinox (spring equinox, march equinox, or northward equinox) is the equinox at the beginning of spring in the northern hemisphere: the moment when the sun appears to cross the celestial equator, heading northward. The equinox occurs around March 20-22, varying slightly each year according to the 400 year cycle of leap years in the Gregorian Calendar. At the present time, the vernal equinox occurs as the sun moves through the constellation Pisces. 2000 years ago the equinox was in Aries and by 2600 it will be in Aquarius.
In the southern hemisphere, the equinox occurs at the same moment, but at the beginning of autumn. There are two conventions for dealing with this: either the name of the equinox can be changed to the autumnal equinox, or (apparently more commonly) the name is unchanged and it is accepted that it is out of sync with the season. The alternative terms March equinox or northward equinox avoid any such ambiguity.
At the equinox, the sun rises directly in the east and sets directly in the west. In the northern hemisphere, before the vernal equinox, the sun rises and sets more and more to the south, and afterwards, it rises and sets more and more to the north.
This is when the Neopagan Sabbat of Ostara (or Eostar) is celebrated. Also, Vernal Equinox Day (春分の日) is an official national holiday in Japan, and is spent visiting family graves, and holding family reunions.
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