On this special date, the length of the day and night are about the same for most of the planet. The amount of solar energy delivered to the Northern and Southern Hemisphere is also equal.
What happens on March 20 is that the sun moves from south of the celestial equator to north of the equator. The days, which have been getting longer since winter solstice on Dec. 21, now equal the nights in length, which is why this is called the "equinox" — Latin for "equal nights."
From now until the summer solstice on June 21, the days will gradually get longer because the sun will be higher in the sky, giving more hours of daylight. As spring wears on, the Northern Hemisphere will receive more sun than the Southern Hemisphere, creating the familiar seasons in each region.
What happens in the Southern Hemisphere when the sun crosses the equator into the northern sky? There the days grow shorter and the temperature drops. Today is in fact the southern autumnal equinox, heading towards winter. The seasons in the Southern Hemisphere are a mirror image of the ones in the north.
Anyway, enjoy the longer days and warmer weather.
Extracts from : www.space.com