The March equinox occurs at 16:15 UTC. The Sun will shine directly on the equator and there will be nearly equal amounts of day and night throughout the world. This is also the first day of spring (vernal equinox) in the Northern Hemisphere and the first day of fall (autumnal equinox) in the Southern Hemisphere.
Many cultures around the world celebrate the whole day as the March equinox. However, in reality, the equinox occurs at a specific moment in time when the Sun crosses the celestial equator, the imaginary line in the sky above Earth’s Equator, from south to north. At this moment, Earth’s axis is tilted neither away from nor towards the Sun, but is rather perpendicular to the Sun’s rays, like the illustration shows.
In 2018, this is on March 20, at 16:15 UTC. For locations that are at more than 10 hours behind UTC, it was on March 19. This is because of the time zone difference.