Each of the four seasons begins at either an equinox or solstice. An Equinox marks the alignment of the equator and ecliptic when day and night are of equal length. A Solstice occurs when the Earth is closest to or at its greatest distance from the Sun.
Spring (vernal) equinox - 21 March
ummer solstice - 21 June
Autumn equinox - 23 Sept
Winter solstice - 22 Dec