Equinox defines the two dates each year at which day and night are of equal length. A more useful set of dates for gardeners are the dates at which the day starts and stops being longer than 10 hours, because this is the minimum number of daylight hours required to allow growth of most plants. I call this the veginox. Here in the Seattle area, at latitude 47N37, the spring veginox is February 8 and the winter veginox is November 3.
You may determine the dates for veginox at your location by using this form.