Despite the temperamental and erratic weather indicating otherwise, the first day of spring hasn't actually happened yet. On Sunday, March 20, 2016, at 12:30 a.m. Eastern Daylight Time, the vernal equinox will occur in the Northern Hemisphere, and the autumnal equinox will occur in the Southern Hemisphere.
What the hell does this mean, you ask? The equinox is derived from the Latin words for aequus (equal) and noctis or nox (night). As such, the equinox is when nighttime hours are approximately equal to daytime hours around the world.
This occurs because of the Earth's axis of rotation; "Instead of a tilt away from or toward the sun, the Earth's axis of rotation is perpendicular to the line connecting the centers of the Earth and the sun during an equinox." In other words, the duration of night and day is equal to approximately 12 hours long across the world.
Equinox shouldn't be confused with another astronomical event; the solstice. Occurring in June and December, the solstice refers to when the Earth's axis is either tilted toward or away from the sun, which results in the longest day and shortest day in the year, depending on whether you're in the Northern or Southern hemispheres.