The March equinox occurs this year at 16:15 UTC on March 20. This is the moment when the subsolar point crosses the equator headed north. The line of day and night as seen on a map as above (produced by Earth and Moon Viewer using NASA imagery with snow and ice cover representative of March) is vertical, indicating that day and night are of equal duration everywhere on Earth. The terminator (day/night line, not killer robots from the future) appears to curve near the poles because of the projection of the spherical Earth onto a flat map. Here is a view of the Earth above the terminator at the moment of equinox.
Astronomers reckon sidereal time (time measured by the positions of the fixed stars as opposed to the Sun) as starting at the March equinox. Due to precession, the date of the equinox, measured in absolute terms by Julian date, changes slowly over time, completing one full revolution every 26,000 years. The date and time of the equinox measured by civil calendars varies from year to year and can occur on March 19, 20, or 21. This is due to slippage caused by leap year adjustment in the Gregorian calendar.