National Weather Service United States Department of Commerce

Winter officially ends at 4:37 am CDT on Saturday, March 20th, with the arrival of the vernal equinox and the start of spring in the Northern Hemisphere. 

During the equinox, the geometric center of the Sun's disk crosses the Equator, and days in the Northern Hemisphere become longer than the nights. The word "equinox" is derived from the Latin aequinoctium, meaning "equal night".  However, despite common belief, the day of the equinox does not feature exactly 12 hours of daylight and darkness, as it is a function of solar refraction and an observer's latitude.  The particular date of equal days and nights (referred to as "equilux") is actually a few days before the vernal equinox.