The start of spring is marked by the vernal equinox, the moment in which the sun crosses the celestial equator going from north to south on the globe.
It is a day that gives way to 12 hours of sunlight and 12 hours of darkness on every point of the globe.
This year it is a moment which occurs on different days in different parts of the U.S.
The exact moment in which spring begins happens at 12:30 a.m. Eastern time on Sunday, March 20. This means that residents in Central, Mountain and Pacific time zones have spring begin on Saturday, March 19. People in Central time will observe the start of spring at 11:30 p.m. CT, and spring will begin at 10:30 p.m. for those in Mountain time. Spring will start on Saturday at 9:30 p.m. for those in Pacific time.
This is not the first time that this has happened, but it certainly is not common.
Aside from the year 2012, it hasn’t occurred in the continental U.S. since 1979. It won't happen again in the continental U.S. until 2037.