Spring equinox is the day of every year when the length (duration) of the day equals approximately the length (duration) of night.
For 2018 (for Milton, Delaware):
January 24 - first day > 10 hours daylight
February 21 - first day > 11 hours daylight
March 17 (Spring Equinox) - 12 hours daylight
Search the internet for "How much sunlight do vegetables need" and you'll find a number of pdf's (here's one) about the different requirements of different crops.