Solstice and Equinox

The Solstice is the day of the year (happens twice a year) when the Earth is at the farthest point from the celestial equator. It also marks symbolically the start/end of specific seasons (specifically the start of winter/summer)

Equinox is the day of the year (happens twice a year) when the Earth has the same amount of night and day time hours.

2008:
Equinox Mar - 20th
Solstice June - 20th
Equinox Sept - 22nd
Solstice Dec - 21st

2009:
Equinox Mar - 20th
Solstice June - 21st
Equinox Sept - 22nd
Solstice Dec - 21st

2010:
Equinox Mar - 20th
Solstice June - 21st
Equinox Sept - 23rd
Solstice Dec - 21st

More here : http://en.wikipedia.org/wiki/Solstice
and
http://aa.usno.navy.mil/data/docs/EarthSeasons.php