Calendar

The date (near March 21 in the northern hemisphere) when night and day are nearly the same length and Sun crosses the celestial equator (i.e., declination 0) moving northward. In the southern hemisphere, the vernal equinox corresponds to the center of the Sun crossing the celestial equator moving southward and occurs on the date of the northern autumnal equinox. The vernal equinox marks the first day of the season of spring.

http://scienceworld.wolfram.com/astronomy/VernalEquinox.html

In the northern hemisphere, the longest day of the year (near June 22) when the Sun is farthest north. In the southern hemisphere, winter and summer solstices are exchanged. The summer solstice marks the first day of the season of summer. The declination of the Sun on the (northern) summer solstice is known as the tropic of cancer (23° 27′).

http://scienceworld.wolfram.com/astronomy/SummerSolstice.html