First Day of Spring
Spring Equinox

March

 (3/21/07, 3/20/08, 3/20/09, 3/20/10, 3/20/11, 3/20/12, 3/20/13, 3/20/14, 3/20/15, 3/20/16, 3/20/17, 3/20/18, 3/20/19, 3/20/20)
 

First Day of Summer
Summer Solstice - longest day of the year

June

(6/21/07, 6/20/08, 6/21/09, 6/21/10, 6/21/11, 6/20/12, 6/21/13, 6/21/14, 6/21/15, 6/20/16, 6/21/17, 6/21/18, 6/21/19, 6/20/20)
 

First Day of Autumn
Fall Equinox - day when night and day are of equal length
 

September

(9/23/07, 9/22/08, 9/22/09, 9/23/10, 9/23/11, 9/22/12, 9/22/13, 9/23/14, 9/23/15, 9/22/16, 9/22/17, 9/23/18, 9/23/19, 9/22/20)
 

First Day of Winter
Winter Solstice - shortest day of the year
 

December

(12/22/06, 12/22/07, 12/21/08, 12/21/09, 12/21/10, 12/22/11, 12/21/12, 12/21/13, 12/21/14, 12/22/15, 12/21/16, 12/21/17, 12/21/18, 12/22/19, 12/21/20)