Spring Equinox 23rd September
This year it will occur at 3.44am NZDT on Tuesday the 23rd.
The word equinox means "equal night": the duration of the day being equal to that of the night, an event which occurs but twice a year, the one marking the astrological beginning of spring and the other the ending of summer. Equal day/night always occurs a few days on the winter side of both the spring and autumn equinoxes.
The Southern Hemisphere Spring Equinox occurs on the 23rd September 2009.