Around 9:37 UTC on Saturday, March 20, the Sun crosses the celestial equator toward the north. This moment marks the Northern Hemisphere's spring equinox and the the first day of astronomical spring. The March equinox s also the Southerner Hemisphere's fall or autumnal equinox and kickstarts the South's fall season.
This site explains in detail how this works and also tells when the March Equinox occurs where the viewer of the site lives.