There is no first day of spring. There is a spring equinox. This spring of 2019 it will be March 20th at 4:58 PM Central Daylight Time (future tense as of time of writing). Adjust accordingly for your time zone.
Equinox is when the days and nights are of equal length. There’s two equinoxes: Spring and Autumn. It happens to be when Earth passes a particular point in orbit around the sun. This year, that moment will be at March 20th and 4:58 PM CDT. After that moment we enter the spring season, and days in the Northern Hemisphere will officially be longer than nights.
I recognize this is the Internet and many will read this after the fact, but the principle remains the same. There is no official first day of spring. There is only the moment of the spring equinox.